I have similar (but worse?) problems. There is missing artwork for many of my albums in iMatch. I have a collection of >15K: everything I've bought all my life, mostly loaded from CDs, then most recently bought from iTunes. On my MBP these albums show up with the "2 notes icon" (i.e. no cover image). On my iPhone5, things show up with diagonal text showing artist and album name, the iOS default when there is no cover image. I tried this "fix" ("workaround"?) but my MBP and iPhone5 still show no cover artwork (for Patsy Cline "12 Greatest Hits", for example).
In addition, I have noticed content glitch(es), and I wonder if others have also? Specifically, in Patsy Cline's album "12 Greatest Hits", at 0:36 of "She's Got You" there is a loud click/scratch sound. I matched this from my own CD on an older MBP. When I play that original, there is no click/scratch. On every device (including newer MBP) when I stream from iTunes store, the click/scratch is there. Seems to me that the Apple iTunes store "master" has been corrupted?!? That is worrisome. Does that mean I have to go through all 15K+ songs to make sure they have not been corrupted? I notified Apple of this problem several weeks ago, but the problem persists. Is all of our music at risk of being corrupted?!?