The Photo Junk module scans your Photos library (or iPhoto, if you haven’t upgraded to Photos yet) to remove useless junk only. In Photos, it’s iCloud local copies, Faces cache, and other app-related cache. In iPhoto, it’s invisible original versions of files that were automatically or manually modified. For instance, if you imported a photo from your camera and then rotated and cropped it, iPhoto will keep an unrotated, uncropped copy of it as a backup. You can remove these unnecessary files to decrease the size of your photo library significantly.