It's incredible how Apple cannot get this right. After each OS update the artwork of my songs don't show up anymore in Finder, in the iTunes Artwork Screensaver, and stop synchronizing correctly with iTunes Match. Anyone gets a quick fix for that? I can make a folder works sometimes by doing randon stuff like renaming the folder in Finder, then renaming the music info in iTunes, then re-renaming... blablabla, but now it's my full library that is not working.
For me, the solution that worked:
1. Get back a 'iTunes Library.itl' file from before the update with Time Machine. This restored the thumbnail from when we click on a song in Finder and the iTunes Screen Saver, but the files themselves still don't have any thumbnail.
2. Thanks to iTunes Match, delete all the music from the library (it doesn't remove any rating, comment or playlist) and download all the files again.
Now everything is back there, all thumbnails in any case. Probably until the next update of ITunes or OS X.