sdpitbull is correct. That's not an option with Mt. Lion. You can send a feature request to Apple via http://www.apple.com/feedback/macosx.html
For some reason, I'm unable to select my iPhoto library as a collection in screensaver options. I've moved my library over from an old machine, and all works well except for this so far...
Brand new iMac with OS X 10.8.2 and iPhoto '11.
I don't think this is possiible with OS X 10.8.2 - I don't have that option on my Screensaver preference to even ask for it to look in my iPhoto library for a background.