How to customize screensaver settings on Login Screen

Automatic login is disabled, so much of the time the family iMac sits at the login screen. I've customized the desktop wallpaper, but I'd like to be able to configure when the screensaver kicks in, and specify an iPhoto slideshow as the screensaver. I've Googled this and searched this group, and have found plenty of entries on customizing the desktop wallpaper, but nothing on customizing the screensaver. Any suggestions greatly appreciated.