Collect and Organize the Images
The first step to creating your custom screen saver is to gather together the images you want to appear onscreen. You do this by creating a photo album within iPhoto. Follow these steps:
In iPhoto, click the Create New Album button.
In the New Album dialog box that appears, type in a name for your screen saver. It can be anything you like. Then click OK.
The new album appears at the bottom of the list, right above the Trash.
Drag photos from your Photo Library, Last Import, or other albums to the album you just created. Choose your favorite photosthe ones you won't mind seeing every time your computer is idle for a while.
For best results, try to stick with landscape or horizontal images. Portrait or vertical images don't work quite as well in a screen saver, although they are acceptable.
When you're finished collecting images, you can select the name of your screen saver album to see all the photos within it. If you want the images to appear in a specific order, drag them around so they're in the order you want them to appear.
When the album is just the way you want it, you're done with iPhoto. Choose Quit from the iPhoto menu.