When it comes to displaying the background of an element outside of that element, there are a few tricks you can try:
body::after{
content:'';
display:block;
position:absolute;
width:100%;
height:100%;
background:url(images/yourimage)
};
If you want to have multiple images on the same element, separate them with commas like so:
background: #color1 background:url(path-to-image1) 0px 0px no-repeat, #color2 background:url(path-to-image) 1px 1px repeat-x; /*and so on*/
Just remember, you cannot make the background image transparent - only the background color. If you need a transparent background image, adjust the opacity of the element instead. To achieve a transparent background color, use:
background:rgba(255,255,255,0.5); /*first 3 values are color codes (RGB), last value is opacity from 0 to 1*/