From Abhisoft Technologies:
Automatic Wallpaper Changer Plus is a tool to automatically change your desktop wallpaper at specified intervals. You can select any number of images and even a folder containing images to be displayed as wallpaper, and set the time interval from every 3 minutes to 6 hours. The program is easily accessible from the system tray. Supported Image formats are .JPG, .BMP, .GIF, .PNG.