Hello,
If you are willing to give up the desktop entirely...
Get TinkerTool and turn off the desktop and then run SaverLab.
SaverLab is a screen saver hosting application that can go background fullscreen.
If the system supplied modules do not fit your needs then do a search on screen saver in Version Tracker (where the above two apps are available).
Some savers drain too much CPU to keep them going all the time. I imagine an image save will use very little.
If you still cannot find one then get one that is open source and write one yourself. You'll be amazed how easy it is with Cocoa's built in screen saver class.