Gswitch automatically changes your GNOME desktop background every so often. The backgrounds are displayed randomly. All background images and style settings are sourced from your list of backgrounds as configured in "System > Preferences > Appearance". By default, images are switched after 20 minutes. Although the images are shown randomly, every image will be shown at least once before you see any repeats.
AutoWallpaper is a cross-platform desktop wallpaper changing utility. It automatically (or manually) changes the desktop wallpaper at a specified time interval set by the user. It runs in the background with a system tray icon that is visible when the application is running. It is written in Visual C# .NET 2008 (Windows versions) and Qt 4.4.3/4.5.0 (Linux/others).