After seen two nice hacks (one with C Sharp using mono and the other with python) about changing the background of your GNOME Desktop using ~/.gnome2/backgrounds.xml, I've decided to create my own version with Bash script you can get the script from

http://patux.net/downloads/change-background.sh
and
http://primates.ximian.com/~gorozco/cha ... kground.sh

This uses XMLStarlet tool to run.

If you are interested on the other hacks you can get those from
http://mieszkancy.ds.pg.gda.pl/~zdzichu/backchanger.cs and http://richip.dhs.org/change-background.py.

Now you can set a crontab to change the wallpaper every 3 hours for example.

crontab -e to open your crontab file and insert the line:

0 */3 * * * /path to script/change-background.sh