With Mac OS X Leopard and installed iPhoto with a bunch of imported images, you will be able to perform really cool screensaver called mosaic screensaver.
Photos to be transformed as mosaic will be chosen from either your whole library of iPhoto or from one of your selected iPhoto albums.
In case you haven't had your iPhoto Library set up, what you need to do is Open your iPhoto and Create Library…. Or if you have created your Library before, you can Choose Existing… library.