You could check out spymac.com - not specifically apple background images, but their background section has over 850 backgrounds - many of them are apple related. It takes a while to search through them, but the upside of that is you get a lot of cool images on the way.
Webshots Daily has a Beta Tester for Mac. You download the software, and then you can download photos from the site. The beta software is not perfect and has some problems: like you sometimes have to adjust the window--widen or shrink it--to see the images you downloaded from the site. Also after double clicking on the photo I want for wallpaper, I usually have to go into preferences and "fill to screen" or "stretch to screen" to get it as a background.
HOWEVER, in spite of these problems, Webshots still has some of the most beautiful images for wallpaper that I know of. The software is free, and you can download, free, 5 "regular" photos a day.
I'll second this one. I really like their selection. I found several different floral images that matched my iSkin keyboard cover.
BTW, Nny mentioned pixelgirlpresents...somehow or other I ended up with a free subscription to MacHome. They always have these monthly themefests where they show you how to doctor up your mac to look totally different. Anyone actually do it? Pixelgirl seems to be a primo place to get the stuff.
there are some good suggestions above, but for some reason I can't stick with a custom desktop too long.,. i just HAVE to go back to "aqua"... there is something about them that makes them visually interesting enough, but not too busy. i have tried replicating them, but i just ccan't do it. it's just aqua for me!