Not really documented, as far as I know.
Just the way how it has been programmed. “web” folder holds data extracted from musicbrainz, “personal” can hold your own images, according this structure:
/data/albumart/personal/artist/<artist_name>/
/data/albumart/personal/album/<artist_name>/<album>/
Please see this posting : Artist View Images?
As we should not go off topic here.