Believe this or not LCDs can actually suffer a version of "static image burn-in" something about the liquid crystal developing a voltage bias when left on a static image for a long period of time. Happened to the smell workstation at my old job. (auto-log out after 5 min inactivity for security purposes - and it displayed the windows log-in screen - indefinitely) Which, upon logging in, you could still read the text from the log-in screen on bright images.
That said, I have Desksaver Plus - which is the widget that I think preceded wallsaver - (does the same thing) And my favorite screen savers:
Abaraesque (Leopard)
Flurry (Leopard)
Shell (Leopard)
Anemona (apple.com downloads)
Filigree (don't remember, sorry)
Soundstream (apple.com downloads)
And Scattered Flurries (apple.com downloads) - set up to project over my current desktop image. Use that with the Strange World II picture, with the wind feature activated in the saver - and the Blimps appear to be moving through the clouds. Messes with my eyes - in a fun way.
All of those seem to play well with anything going on my iMac. (no slow-downs or jittery screen savers)