There's also http://gnome-look.org/
, as well as http://www.wallpapers.com/index.aspx
I'm partial to nature scenes and cityscapes. I have a little script that I run on my Fluxbox desktops to rotate the wallpapers; I call it from the .fluxbox/startup file:
while true; do
fbsetbg -a -R ~/[path-to]/wallpapers
# fbsetbg -a -R ~/[path-to]/Christmas
The part that is remarked out is the path to my holiday wallpapers.
It was adapted from an FVWN script that I found.