I'm sure I'm just doing something wrong but I can't figure out what. I think I have configured MusicBee to expect album art to appear as a file called folder.jpg in the same folder as the music file. You can see to the right a folder with a folder.jpg file. And you can see at the top that MusicBee is not finding the album art. (Yes, I did rescan.)
The problem is not particular to this one folder.jpg file. I have thousands of music files and a lot of folder.jpg files, and at the moment none of them is appearing in MusicBee.
This used to work and I'm sure it will again. I just moved all my files to a new computer and installed MusicBee there, and that's when the trouble started. So it's very likely that I'm just missing some configuration setting on the new machine.