Hey everyone,
I've been fighting with iTunes match to update my album art the way I wanted it to, but found it always reverted back to the art I originally attached to the file. On some occasions it would stay for a while, and then revert back once Match updated.
I believe I have found the fix.
1. Turn off match on all devices except for the computer you manage your music from (this step isn't entirely needed but I have found it gives best results).
2. Select the album/track that needs album art/editing, and select Get Info.
If album, select the check box and OK to clear the attached album art.
If single track, go the album art tab and delete it from there.
3. Here's where it diverts a bit depending on how you manage your music:
If you have a local copy of your songs stored on your system (in the iTunes music folder), when you highlight the album/track and paste in your new/fixed album art, it's going to attach the art to your LOCAL mp3 files. The cloud mp3 file is unaffected (most of the time).
If you keep your music stored entirely in the cloud (as I do, for the most part), where there are no local copies, when you attach the new/edited album art, you are then going to DOWNLOAD the local copy, where it will end up in your iTunes music folder.
4. In both of these cases, you now have a local copy of your album with the new/edited album art attached.
Now, you are going to select the album/track that you edited, and delete it from iCloud. Choose the option, when prompted, to KEEP THE FILES.
5. Since you now have brand new files for Match to deal with, you are going to go into your iTunes music folder and drag your local copy of the album/track you've been working with. Match will see it as new and upload it accordingly.
6. Turn match on to your other devices, see results.
This has been working for me, I usually will Google search the album and sort/find the highest resolution/best looking image and attach it, since I like the art to show up clear on my iPad as well (and sometimes I will find an alternate album cover I wasn't aware of and want to change what I had attached).