DC: I'll make some assumptions:
1) You have beryl started (Cassandra -> system tools -> beryl Manager.
2) you have right clicked the diamond in the sys-tray and selected the window manager to be beryl.
3) opened the beryl settings manager and have the manger panel open. And you select Desktop.
From the desktop panel you can select Desktop Cube, Rotate Cube, etc.
4) Now for the big assumption you want to have a background picture behind the rotating cube? For that you select the Skydome tab on the Desktop Cube panel and then type or browse to the path to the the image you want to use in the space provided.
If I have missed with all the assumptions post back with more information.