For some time I've been trying to put a simple background image behind my daughter's blog. I can't, and a cursory romp through Google suggests that nobody else can, either.
In fact, the background comes out plain white, no matter what anybody does, including whoever wrote twentytwelve. If you plod through style.css, you'll see many, many background statements, with elaborate gradients for background color, etc. None of it appears on the screen.
Sorry, I haven't promoted this stuff to my daughter's site yet. Still hammering away on localhost.
Anybody else curious about this?