As for a new background, as far as I can tell, any background image wouldn't be very visible. You see the thin grey border around the white square that takes up most of the screen and is the background for the majority of the page? That thin grey border is actually the true background. Not too visible, is it?
However, the board's colour scheme is totally customizable. I've got colour options out the wazoo that can be changed around.
Also, some images that *could* be nicely customized? These guys, for example.
All those little icons that mark threads and whatnot, I can replace. So anybody that likes icon-making could come up with some dragony icons to use in place of those, and I can edit them in. Course, those should probably be done *after* a board scheme is developed, unless everyone is totally happy with the colour scheme as is.
Here's an example of a phpBB board that's been customized:
http://www.creativeinsanity.net/stuff/tryne/index.php