If I have a folder of music from a particular album, we know using folder.jpg gives that folder of music and all the songs inside the thumbnail of that image.

And if we add a different thumbnail for a particular song,
thissong.tbn to go with thissong.mp3
that song will use the different thumbnail image

The problem is that XBMC doesn't check for a song's tbn file before it goes to the "global" folder.jpg, defeating the entire purpose of making the tbn files in the first place, and there's no "refresh thumbnails" option like in the Pictures section.