Hi Richard,
Backgrounds are a little more difficult. This is mostly due to us supporting IE6 for our current layouts. Several of our layouts have 'shadowed' edges. Those edges are a combination of the background color gradiated into black. Therefore, the background has to remain that color and with no background images, or it will look funny.
Hope that didin't confuse you ;). In your case, the layout you are using, this wouldn't be a problem. You can upload a background image (let's say you upload one called 'bg.jpg') and use the following code to tile it on your layouts background:
body{background: url(bg.jpg);}