WallpaperFlickr

If you're more picky than just that, you can choose pictures using a keyword search, or photos by a particular user, or favourites for a particular user - you can also vary the time each wallpaper stays on the screen for.

And if you like the pictures, you can visit Flickr for more information on them. What's not to love?
Fed up of gazing at your default desktop wallpaper? Use popular pictures uploaded recently to Flickr as your Windows wallpaper. You'll get new wallpaper every hour from the best photos taken in the last seven days around the world.