GNU/Linux Desktop Survival Guide
by Graham Williams
The Gnome background can be set from the desktop with the right mouse button menu. Choose Change Desktop Background.
Suppose you have a bunch of background images in /home/share/backgrounds. The following command will randomly choose one to display. You could put this as a cron job to run each hour!
$ gconftool -t str -s /desktop/gnome/background/picture_filename \ "`find /home/share/backgrounds | random | tail -n 1`"