Depends on the kind of background. Is it one that can repeat, so that it simply fills the area it's assigned? That's the easiest. If it's a fixed size image as a background that you always want the entire picture to be seen as the window adjust size, then it's trickier. Setting the background with CSS can't do scaling like this, at least not current CSS.
What you need to do it use the img tag, place the image using absolute positioning, and its z-index to 0 and everything else around it to 1. Then on the img you can set the width and height to 100% so it'll always fill that spot. Also, on the parent element it'll need position:relative so that when you're using absolute positioning on the img, it'll be easier to position it.
I'm not saying this will be easy to implement, but it's a solution I've seen used.