I don't think there is any way to do this within the MusicBee app itself and take my suggestion here with due caution as this is the first time I have looked at this (and note that I only use the portable version):
I believe banned pictures are logged in:
Portable version: .\AppData\Banned Pictures.dat
Installer version (this is a guess): %APPDATA%\MusicBee\Banned Pictures.dat
Store version (I have no idea): *****************?\Banned Pictures.dat
You can edit this in a text editor but, as it is a list of web addresses, it is not easy to see what picture each address links to (you can copy the link into a browser to check).
I would suggest making a backup of this file before messing around.