Nine free pet oriented wallpapers avail at Free Pet Projects.
If you don't mind a little invasion of your desktop (they have an organizer), you can get a program that will cycle your wallpapers from Webshots. They offer premium services and ways to share your own images with the community.
Another alternative for changing wallpapers on Vista comes to you from Dancemammal, it has several interesting features and is a pretty slick lil program.
The best way to find wallpaper, however, is to search with your favorite search engine for an image you like. I use Google's Image search along with selecting the largest images possible, you get some pretty good stuff that way. Just remember, most of these images will be copyright, so don't post them to your site without permission.
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