After looking around for something else to do this and not being satisfied with a shell script, I broke out my rusty C# skills and put together a small app myself. When using Xfce, you can use a list of images as desktop wallpaper, and the desktop will load a random image from that list whenever you log in. My program simply sits in your tray and changes your wallpaper on a timed basis rather than only when you log in. The code is nothing special, but it gets the job done.

If you're one of those folk with something against Mono, I've re-coded the app in C/Gtk+ as well, so you can download the Non-Mono Edition if that's the poison you prefer.