This is old news for lots of people, but I've been downloading background pictures ("wallpaper") lately from MacDesktops.com. I don't really know why the guy calls it MacDesktops, as the images should be perfectly usable on any operating system. There are a number of images of Macs and Apple-related things, but for me the landscapes and waterscapes are much more pleasant to look at. Images are in a wide variety of resolutions -- I haven't seen other sites where the images are not only in the standard 4:3 ratio but are also in widescreen resolutions like 1920x1200 and 1680x1250.
Anyway, I like the pictures and sent the guy $20, since I've used a lot of his bandwidth downloading them. Check it out.