If your Aperture slideshow is missing images check to make sure you created previews. I guess sometimes raw files don’t get previews. Now that Flikr is more or less unlimited I’ve started putting my best photos there. My TV screen saver is my best of Flikr shots. Unfortunately Flikr killed the rss feed for sets so there’s no easy way to display them as a screen saver. Aperture & iPhoto should show up in the Slideshow Screen Saver but don’t for some reason. They do show up for Desktop images.