If you want to run a screensaver as a desktop, go to the terminal and type:

/System/Library/Frameworks/ScreenSaver.framework/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine -background &

The terminal will return the process ID (PID), so when you get bored or need the power, you can kill the process by typing

kill (followed by the PID number - no brackets)

Your previous desktop picture will return immediately.

Tom.