Adjust the size of the main image container as needed

.imageContainer {
  width: 800px;
  height: 600px;
}

img {
  width: 100%;
  height: 100%
}

<div class="imageContainer">

  <img src="https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQqdnZQDnZOOyBGXXB23WS87IAxZ85drC-ylgrqWCqYk2aEh3Vo">
</div>

You can use CSS to resize images, either in a style sheet or within the style attribute.

For example, you can stretch a 32x32 gravatar to 128x128 using the following code:

.stretch-128 {
  width: 128px;
  height: 128px;
}

<img src="https://www.gravatar.com/avatar/6b938dc4205cd0cea4e4e68451c42a21?s=32&d=identicon&r=PG" class="stretch-128">

Keep in mind that when you stretch an image like this, the quality may be compromised.

To start, I crafted a div labeled divWrapper with specific dimensions. Inside it, we established a class for our image with a width of 100%, spanning the entire divWapper, and a height set to auto for maintaining proper proportions automatically. The image size can be adjusted based on the divWapper class.

.divWrapper{
width:640px;
height:480px;
float:left;
}
//Now set your image class to 
.imageclass{  
width:100%
height:auto;
float:left;
}

<div class="divWrapper">
<image class="imageclass" src="https://upload.wikimedia.org/wikipedia/commons/2/2d/Picea_glauca_taiga.jpg">
<div>

This method ensures the image stays in proportion regardless of the container size.

Useful Tip:

It's essential to begin with a high-quality image; I recommend using vector images like PNG or SVG for superior quality compared to JPEG and other formats. Here are some links providing insights into different image extensions:

https://www.sitepoint.com/gif-png-jpg-which-one-to-use/

You have 4 options to achieve this task.

1. Setting the width attribute directly for the image

Main principle: adjust the width of the image itself.

Set width: 640px; display: block for the img element.

2. Wrapping the image with a div container

Main principle: inherit the width from the parent element.

1. Division with width: 640px
2. Image with display: block; width: 100%

3. Using the image as background of a div

Main principle: treating the image as a background of an element, not directly as an image tag.

1. A div with background-image: url(), using the image as its background
2. A div with

   background-size: {2x width} {2x height}

   , to resize the background image

4. Scaling the image by 2 times

1. Apply the transform property with scale(2) value to increase the size of the image

Tips: Additionally, set the transform-origin property to top left, ensuring proper alignment from the top-left corner when scaling.

Execute the code snippet above to see the results :)

.method-1 {
  display: block;
  width: 640px;
}

.method-2 {
  width: 640px;
}
.method-2 img {
  display: block;
  width: 100%;
}

.method-3 {
  width: 640px;
  height: 480px;
  background-image: url(http://www.placehold.it/320x240);
  background-size: 640px 480px;
}

.method-4 {
  transform-origin: top left;
  transform: scale(2);
}

<h1>method 1: img displays as block, width 640px</h1>
<img src="http://www.placehold.it/320x240" class="method-1" />

<br/>

<h1>method 2: parent dom width 640px, img width 100%.</h1>
<div class="method-2">
  <img src="http://www.placehold.it/320x240" />
</div>

<br/>

<h1>method 3: the img as div's background, use the background-size attribute to set 2x size.</h1>
<div class="method-3"></div>

<br/>

<h1>method 4: use CSS3 transform property to scale the image into 2x.</h1>
<p>tips: when we set `transform: scale(2);`, we also need to define the `transform-origin` property to `top left`, in order that the image will align from top to bottom as well as from left to right, or it will lost itself. LOL.</p>
<img src="http://www.placehold.it/320x240" class="method-4" />

Check out these tips for effectively loading images:

• Avoid FOUC, which occurs when the browser doesn't know the image size beforehand or is given incorrect sizing information.
• Avoid distortion by displaying images at their native aspect ratio of 1:1.
• Prevent pixelation by not enlarging images significantly beyond their original size.
• Avoid loading large images that are bigger than necessary for the page, as this can slow down the site and waste bandwidth.

There are various methods to address each issue:

To prevent FOUC:

• Save image sizes alongside files to allocate correct space before loading (WordPress does this automatically).
• Load images in containers with background-size: cover to avoid FOUC and broken anchors if images fail to load.
• Animate image scale from 0 to 1 within a centered container to avoid FOUC while preserving design integrity.

Regarding distortion:

Avoid it at all costs, as any alternative like cropping or pixelation is preferable.

About pixelation:

• Add semi-transparent patterns over large background images to reduce noticeable pixelation.
• Display pixelated thumbnail versions below the fold at reduced opacity, lazy-loading full-size images upon scrolling into view for quality presentation.

Avoid oversizing:

Utilize src-set and thumbnail loaders to efficiently load images and prioritize lazy-loading for content below the fold.

In essence, these strategies ensure optimal image loading, although there are additional considerations such as optimization, raster vs vector graphics, and formats like SVGs and WebP.

To accomplish this, you have two options outlined below.

<img class="yy" src="http://placehold.jp/150x150.png">

.yy {
zoom: 200%;
}
.yy {
 transform: scale(2);
}

I've struggled to find a reliable source detailing how to scale images using client-side technologies without sacrificing image quality.

However, if you're looking for the most effective way to double the size of your image while adhering to HTML Standard specifications, it's recommended to use a different image that has already been scaled up through backend processes or editing software like Photoshop. For creating adaptive images based on the user's screen size, following the HTML Standard guidelines is key:

<picture>
 <source srcset="http://www.placehold.it/320x240" media="(min-width: 600px)">
 <source srcset="http://www.placehold.it/640x480" media="(min-width: 900px)">
 <img src="http://www.placehold.it/160x120">
</picture>

In my personal experience, the most effective method to increase the size of an image for use on websites is to double the image using a program like Photoshop before uploading it. Dealing with factors such as image size, pixel density for retina screens, and various screen sizes has made working with images more complex than in the past, but it's not overly difficult.

Consider the example below:

<picture>
  <source
    sizes="(width < 32em) contain, (width >=32em) contain 80vw calc(100vh - 10em)"
    srcset="full.jpg 2048h 1024w, half.jpg 1024h, quarter.jpg 512h, eighth.jpg 256h" />
  <img src="eighth.jpg" alt="rose flower!" />
</picture>

To ensure optimal display quality on all devices, upload a high-resolution image (at least FHD) and use 'sizes' and 'srcset' attributes to determine which version to load based on screen size/type. This approach guarantees that your image will look its best regardless of resolution. Although some manual adjustments are required before uploading each image, the end result justifies the effort!

If you prefer to keep the original small image file and double it through coding, simply add a CSS class to the image and specify the doubled dimensions in that class.

For a humorous yet informative explanation of images srcset, sizes, and why this process is necessary, check out this article: A fun article on srcset

You can utilize JavaScript to easily change the size of an image by doubling it.

Take a look at this JsFiddle demonstration

<html lang="en">
<head>
<script src="https://code.jquery.com/jquery-1.12.4.min.js"></script>
<script type="text/javascript">
$(document).ready(function(){
 $("button").click(function(){
    var img = $("#sky");
    // Create dummy image to get real size
    $("<img>").attr("src", $(img).attr("src")).load(function(){
        var realWidth = this.width;
        var realHeight = this.height;
        alert("Original width=" + realWidth + ", " + "Original height=" +     realHeight);
        var newWidth = realWidth * 2;
        var newHeight = realHeight * 2;
        $("#sky").width(newWidth);
        $("#sky").width(newHeight);
         alert("new Image Size" + newWidth+"x"+newHeight);
    });
});   
});
</script>
</head>
<body>
<img src="https://www.cluedin.net/images/providers/stackoverflow.png" id="sky" width="250" alt="Cloudy Sky" contextmenu="skymenu">
    <p><button type="button">Get Original Image Size And Double it</button></p>
</body>
</html>                            

I appreciate the creativity in the solutions provided so far, but one major challenge remains - we lack information on the current image size required for doubling it.

My approach to this problem is quite dynamic and ensures that other images on your website remain unaffected. Moreover, it can easily be modified to increase the size of images by tripling or even more if desired!

Take a look at the Live Preview here.

You can also find the code below:

HTML

<img class="resize-double" src="https://placeimg.com/100/50/nature">
<img src="https://placeimg.com/200/100/nature">

JavaScript

// Identify all images to resize
const toResizeImgs = document.querySelectorAll(".resize-double");

// Resize each image by doubling its size
for(let i=0; i < toResizeImgs.length; i++) {
  toResizeImgs[i].style.width = toResizeImgs[i].offsetWidth * 2 + "px";
  toResizeImgs[i].style.height = toResizeImgs[i].offsetHeight + "px";
}

This code snippet targets images with the class resize-double, iterates through them, and adjusts their width/height to double the existing dimensions!

NOTE: The use of offsetWidth and offsetHeight accounts for Margins, Paddings, and Borders when determining the current size. Alternatively, you can utilize clientWidth and clientHeight, or even scrollWidth and scrollHeight based on your specific requirements!

The most efficient method involves uploading the largest image onto your website and then reducing its size using CSS as needed.

1) This ensures the best quality for your image as upscaling can distort it.

2) There is no requirement to upload multiple images with different sizes.

3) Downscaling does not degrade the image quality and only requires a few lines of CSS code.

If you prefer not to use the larger image, the second best and quickest option is to upscale the image using CSS. The width attribute no longer functions in <img> tags.

There are numerous ways to achieve this task. However, it is crucial to download the image at the desired size before making any adjustments. If you simply double the size without downloading it first, the image may appear pixelated or faded.

  image{ transform: scale(2); }

1. Consider using transform scale to double the size of your image

2. Explore setting the width of the image using CSS

3. Experiment with JavaScript and use setAttribute("width", "size")

One way to prevent stretching is by employing CSS.

img{ width:640px; background-size: cover; height:auto;}

By using this code, your image will stretch proportionally with an appropriate height. Another option is to set a specific height for the image.

It is important to note that the dimension attributes should not be used to stretch the image unnecessarily.

Although using the width attribute may not stretch the image as intended, a good alternative is to use transform: scale(2);. However, this method can sometimes result in image overlap with other content. In such cases, it is recommended to adjust the width of the containing div to accommodate the scaled image with the CSS property 'transform: scale(2);'

Therefore: implementing the following style would be a better approach.

.img2x_container {
    width: 640px;
    height: 480px;
}

.img2x_container img {
    transform: scale(2);
}

This technique is most useful when the dimensions of the image are known in advance.

A helpful technique is to utilize transform -> scale

img#imagId {
    -ms-transform: scale(2); /* IE 9 */
    -webkit-transform: scale(2); /* Safari 3-8 */
    transform: scale(2);
}

Below is the Java Script code I use to resize images to fit perfectly within a designated container. Feel free to customize it as needed.

function adjustImage(container, image) {
var c = document.getElementById(container);
var i = document.getElementById(image);
var newSize = determineSize(c.clientWidth, c.clientHeight, i.width, i.height);
i.style.width = newSize.width;
i.style.height = newSize.height;
c.style.backgroundColor = "transparent";
c.style.border = "none";
var intHeight = ((100-((parseFloat(i.style.height)/87)*100))/2);
var strHeightPercentage = intHeight.toString().concat("%");
c.style.top = strHeightPercentage;}

function determineSize(maxW, maxH, currW, currH) { var ratio = currH / currW;

if (currW >= maxW && ratio <= 1) {
    currW = maxW;
    currH = currW * ratio;
}
else if (currH >= maxH) {
    currH = maxH;
    currW = currH / ratio;
}
return { width: currW, height: currH };

}

Clever CSS Hack for Adjusting Image Size

When it comes to using CSS to adjust image sizes, there's a handy trick you can try:

img#image_id {
    width: calc(2 * 320px);
    height: auto; /* This line is optional */
}

This technique allows you to easily modify the size of an image by simply adjusting one value (the factor). The browser will automatically set the image height even if you don't explicitly specify it as 'auto' is the default value for both width and height properties (refer to this link and this link). Alternatively, you can manipulate the height instead of the width.

It can be particularly handy for responsive design purposes, adding a touch of complexity to your styling techniques!

Keep in Mind: This method may not work as expected with flex-boxes when setting the parent's dimensions. In such cases, you'll need to explicitly define the width and height values or seek other solutions since flex-box behavior differs significantly.