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
03.jpg
...
etc.
Keep in mind the Apple TV has 8GB internal storage. As far as I know, it's pretty hard to come close to that with screensaver sets (EDIT: see below). Looking for a good collection? FiftyFootShadows has a nice collection of Retina iPad wallpapers that look great (the ATV scales photos automatically for wallpapers).
EDIT If you're feeling particularly savvy, SSH into your ATV and run df -h. You'll see something like this.
Filesystem Size Used Avail Use% Mounted on
/dev/disk0s1s1 681M 598M 76M 89% /
devfs 26K 26K 0 100% /dev
/dev/disk0s1s2 6.8G 417M 6.4G 7% /private/var
The top one (/) is the one you want. At 40 images, it fills up to about 90%. Currently trying to fix this, I'll tweet this again if I find a good solution.
Go forth and do hackery.
-kristian