serge@cyberlab:~$ crontab -l
# m h dom mon dow command
0 0-23/4 * * * ~/bin/set_background
serge@cyberlab:~$ cat ~/bin/set_background
#! /bin/bash
cd ~/pics
wget http://tools.wikimedia.de/~daniel/potd/
commons/potd-1280x1024.jpg
gconftool -t string --set /desktop/gnome/background/picture_filename
~/pics/potd-1280x1024.jpg
serge@cyberlab:~$