Here is another useful tool that I have only recently become aware of. The Wallch wallpaper changer is a simple wallpaper manager with some extra helpful features. Wallch will work on Ubuntu’s new Unity desktop or the Gnome Shell desktop.
About Wallch Wallpaper Changer
Use this tool to randomly switch through your list of available wallpapers at a desired interval. You can even check out a preview to see how it will look.