Is it the new AppleTV or old?
The old can copy your entire library onto its hard drive and can randomly show photos from your entire library during the screensaver. However, I have noticed that it is not totally random. It will tend to repeat photos from a certain event, but still seems random enough, showing both new and old photos.
The new AppleTV cannot access your entire library, it can only access a single album or event. I use the album of photos from the last 12 months. I guess you could create an album and put your whole library into it. However, that may cause problems. I think that I have read that the iTunes reduces the resolution of your photos to match the resolution of your AppleTV then stores those smaller photos on the flash memory of the AppleTV -- so that the AppleTV screen saver will work even when not connected. I would be concerned that a huge photo library might eat up too much of your 8 gigs needed for streaming etc. Maybe there is a limit on the number of photos that can be transfered so that streaming is not effected.