There is the best little free program called webshots. I think you have to sign up, but it's free and you don't get junk mail. They have thousands of walls for you to download, and you can import your own also. Webshots it IMHO the *best* place to share your own photos.
With the free membership you can only download 5 photos per day (but by the sounds of it, I get the impression it's walls you allready have you wan't to use anyway), but I don't recall any spyware, adware or anything of that sort. You can also use webshots as a screensaver and it has a ton of other cool wee features like being able to display your pics in B/W and adding a calander.
Check it out, you might even consider upgrading your mebership once youve gotten into it! (i dunno how much it is, its a sub tho)
http://www.webshots.com