The Apple TV has some nice pictures for screensavers, but there's only about ~20 on average for each screensaver set. Luckily, with a jailbroken Apple TV, we can replace them pretty easily.
The screensaver images are located at /System/Library/PrivateFrameworks/AppleTV.framework/Default[Album]Photos
. I like SFTP'ing in with Cyberduck.
There are multiple screensaver sets — if you have a set you like the least (maybe "FlowerPhotos"?), replace those.
The files are in a pretty basic format:
01.jpg
02.jpg