I suffered the same problem as Force Google Music Player to refresh album art? but I fixed it manually. That is, downloaded a local copy of the music on my phone and manually updated each MP3's album art. However, the web copy is still bad.
If I "upload" the fixed mp3 files will it update the files rather than upload a duplicate of it?
I don't want to try and have the latter happen and have a huge mess to clean up.