Hello.
I picked up and tweaked up some python code (http://symbolx.org/projects/code/gnome-bg-randomizer.txt) I found on the web that randomizes Gnome's background image similarly to how OS X does it.
Just download it, place it somewhere, and setup a cron job to run the script every so often. Make sure you modify the BG_DIR variable in it to point to a directory relative to your home directory that contains images.
In case earlier link is hard to see, it's here: http://symbolx.org/projects/code/gnome-bg-randomizer.txt (http://symbolx.org/projects/code/gnome-bg-randomizer.txt)