I'm not hip to CSS, but i figure it must be somewhat simple to change the code
to allow an image to be displayed so that it fills the entire browser width?
If you want the background to cover the screen you should use a small image that tiles nicely, large wall-paper sized images load slowly and use a lot of bandwidth. Avoid using something you might post on your desktop as a background image
If you have a small image and you want it tile the code would look something like