I haven't yet tried Infinite Walls (free), but it seems like a great idea. The app lets you search Google Images for photos of subjects that interest you. The app automatically resizes the files for use as wallpapers on your iPhone, iPad, or iPod touch, automatically detecting which device you're using. In-app purchases let you add more features, such as a vault where you can store images and have access to them without an Internet connection.
Infinite Walls
