All of your media files (images, videos, audio, text, HTML, etc.) are stored in the pool folder. Their names are changed to match the hash keys that BrightAuthor generates when the files are published. If you look in the current-sync.xml file or the local-sync.xml file, you'll see a listing of all files and their matching hash keys.
Files in the pool folder are pruned automatically. The player only deletes files when it's starting to run out of space. Any files that aren't part of a currently scheduled presentation can be potentially deleted when more space is needed for the next scheduled download.