Inside \MusicBee\AppData\InternalCache, there are cached album covers...artist backdrops, data and thumbs.
It is safe to remove those from time to time, since MB will just recreate them the next time you use the application.
But what you should also know is that when the subfolder 'ArtistData' is removed, it takes with it all the changes/edits that you made to artist biographies.
A large number of my artist bios are edited (some of the web-retrieved are usually not enough for my liking).
I nearly learnt the hard way...fortunately, I hadn't emptied the bin by the time I discovered what was up (but I had backups too anyway).