Windows only: Wallpaper Shifter is a lightweight wallpaper randomiser. The installation is 8.11MB which by portable-application standards isn't super light, but the system resources consumed by Walllpaper Shifter are extremely lightweight.
Wallpaper Shifter is a little program that starts when you login to Windows and picks a random wallpaper from your list. Depending on the wallpaper width and height, it can automagically scale it for you to fit your desktop without deforming it's proportions. It does not remain running in the background so no system resources are used.
If you need a more complex wallpaper changer that will stay active during your Window session, check out John's Background Switcher. Wallpaper Shifter is freeware, Windows only.