This feature exists in CSS too. It's called background-image and lets you define one or more images to be displayed as a background of a container.
background-image: url(front.png), url(behind.png);
Remember to make the container a block element and if you don't have any content in it you should set the container height and width or a padding-bottom to give it size (Otherwise you will be shown approximately 0px of your background). See http://stackoverflow.com/questions/1495407/css-a-way-to-maintain-aspect-ratio-when-resizing-a-div