Latest round of experiments...
Amazingly, Gracenote does use some fuzzy logic on matching the song to the cover. I tried it with Jimmy Buffet (s/b two "t"s) and it still worked. Tried it with Fruitcake (missing the "s") and it still worked. No idea how far it will go, but it clearly doesn't have to be perfect.
Then I started fiddling with embedded cover art. I used everything from 300x300 up to 900x900, with several in-between. They all worked. I didn't see any difference in display quality, so anything over 300x300 is probably a waste. I even tried an 1100x900, to see how it handled non-square images. It worked too, without any noticeable stretching.
I even took a pic of my wife and embedded it. Also worked as expected. This creates the ability to have some fun with cover art.
The only drawback to tweaking my own music is the inability to access Gracenote's db. When I come across MP3s that don't display a cover, if I could get to their db, I could figure out what I need to change. Without that access, I will have to embed JPGs to get cover art.
2013 550i Space Gray, M-Sport--del Dec 2013
2013 X5 5.0 Orion Silver--del Dec 2013
2009 X5 4.8 White (w/hitch)
2007 550i Monaco Blue, Sport
2004 X5 4.4i
2003 540i M-Sport w/Dinan mods (last of the great E39's)
2002 X5 3.0
2000 540i Sport
1995 525i Sport