Is your music library consolidated into the iTunes media folder? If you go to the Preferences, and in Advanced Preferences, you an select "Copy Songs to iTunes Media Folder when adding to Library" ... that will put all the music where iTunes can find it easily. My artwork all reappeared in Finder when I did the following simple step:
1) In the general preferences, select "Automatically Find Missing Artwork"
2) Go to the File menu and select Library (the one with the arrow). From the arrow choices, select Organize Library, and iTunes will make a copy of every song used by iTunes into the Media Folder. (iTunes does not move your original files, but if your Media Folder is up to date, you shouldn't need them except as a backup.) This should restore most of your missing artwork. You can always add the few missing or erroneous art files yourself, and when you do it This way, they will no longer be missing, because they are in the media folder.
🙂 Hope this helps. It was driving me mad, now my artwork is all repaired and I'm happy.
And YES - I am now seeing my artwork in the finder, and viewer as well.