<img class='background' src='sourcefile'>
Oh, in the subject line you write div width and in the body you write screen width. The above is for screen width but I'm sure you can adjust to also fit it in a div, although the div width may be decided by the image width.
Vary the width of the window and see the text size change.
You'll see that the banner fills much more than the screen; it's a div with 1600% screen width and the banner consists of pictures that are shown in a random order. If you need to you can check the source and download the css file.