In iTunes you should now have a window that looks just like the page in iTunes that manages your photos in iPhone or iPad (if you're familiar). Note, this is on your computer- not your ATV. You can only select on your ATav from the pictures you have authorized back in your iTunes library.
Your pics are organized justnike in iPhoto and you just check off whichever photos files you'd like. You can chooses from Events, Faces, pictures, or all of the above..
I'm not sure about non-PC operating systems but for PC's running Windows operating systems the only folders shown for selection are those that are the next subdirectory below the main folder selected for sharing in the iTunes Photo Sharing setup dialog. For example, I have "Al's Photos" selected for sharing in my iTunes Photo Sharing setup and the only photo folders I see for selection on my Apple TV 3 are those that are the next level down (subdirectory) from "Al's Photos". Many of those subdirectories have multiple folders (directories) below them, however, those subdirectories cannot be individually selected in Apple TV 3. The photos in those 3rd and 4th level subdirectories do, however, appear in the photo slideshows as long as their root directory folder(s) have been selected in Apple TV 3.
If you need to isolate some of your photos by year then I suggest putting them in one or more subdirectories that are only one level lower than your main "shared photos" directory.