I forgot to mention that I just finished the latest version of John’s Background Switcher and have added a bunch of new features (thanks to those who suggested them).
If you haven’t seen it and wonder what I’m talking about, it’s a simple little application that sits in the system tray (down by the clock) of your Windows PC (sorry, I’m afraid it’s Windows-only) and changes the desktop background at specified intervals. You pick the pictures, when you want them to change and if you want them to be picked randomly and it just quietly goes about its business.
It’s completely free and I’m still surprised something like it doesn’t come as part of Windows! You can download it from this page. Enjoy and if you have any suggestions or problems then let me know (I’m working on V2 at the moment).