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.
Most people use default name iPhoto Library as the name of their iPhoto Library. In order to save this library, we can rename the whole folder to something else, perhaps iPhoto Library Old and then open iPhoto to create another iPhoto library, the new one.
It's fairly easy to import images to your iPhoto library, simple drag-n-drop will definitely do the task. Or if you're not used to the process of drag-n-drop, you can use menubar File - Import to Library… (keystrokes: Command-Shift-I).
You will have different photos with different categories and on different occasions. This is when the albums come into assistance. To create new iPhoto album from your existing iPhoto library, you can use menubar File - New Album… (keystrokes Command-N).
Assume that you've followed the flow of the instructions above and now you've got your iPhoto Library and Albums categorization set up. Go inside your System Preferences - Desktop & Screen Saver - Screen Saver. Under your Pictures section, you will see folders with unique icons (Film Roll, Album, etc), these are the iPhoto screensaver; You can choose one of these.
After selecting one of those iPhoto albums, just slightly below the screensaver preview pane, you can see three icons for selecting three different display style. Choose the rightmost one which is the Mosaic Screen Saver.
I've captured dozen of my mosaics and put it on my Flickr Slideshow: Mosaic Screen Savers.
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