The whole of iPhoto?
Not sure on videos, but photos:
System Preferences --> Desktop & Screen Saver --> Screen Savers
In the Screen Savers field on the left, choose iPhoto, or an individual folder from within iPhoto if you want to narrow it down.
Click the '+' button and browse for another folder if it isn't in iPhoto.
Then, once it has loaded them, they will appear in the box on the right, with the three options below that box.