Utilize an image with partial transparency (such as in your illustration, likely in png
format) and position it centrally within a container that is set to position: fixed
and covers the entire window size, as shown below:
.container {
position: fixed;
width: 100vw;
height: 100vh;
display: flex;
justify-content: center;
align-items: center;
}
The remaining content (like your moving background images) should be placed outside of this container.
ADDITION:
To adapt the image size based on the window size, assign a percentage-based size like this:
.container img {
width: 60%;
height: auto;
}
Example: https://codepen.io/anon/pen/mKwZVV (demonstrating text instead of background images as a substitute for a partially transparent placeholder image)