i am using 3d desktop (the cube) with compiz.

is it possible to have different background on each side of the cube when you rotate the cube ?

Help would be very much appreciated !!!

I'm not sure if this is possible with Compiz but it works with Beryl.

how do you do it in beryl ?

To get to know Beryl visit:
http://www.beryl-project.org/userguide.php
http://www.beryl-project.org/faq.php#ub1
and the site in general.

I have not tried it myself so I'm afraid I cannot guide you, but what you need should be the "wallpaper-plugin/Wallpaper manager".
http://wiki.beryl-project.org/wiki/Wallpaper_Manager

I also recommend the forum.
Good luck.