How to change album cover on Apple Music

Okay, in case it helps, this did the trick for me:

1. In iTunes, I unchecked the music sync, and then synced the phone.
2. Confirmed there were no more playlists on the iPhone.
3. Checked the lists of SONGS -- yep, they were still there.
4. On the iPhone went to settings >> iPhone storage and deleted all the songs and playlists.
5. Back in iTunes, did another sync (making sure that the "sync music" was UNCHECKED)
6. Checked the phone -- no songs, no playlists.
7. In iTunes, checked a few TEST playlists on to sync to the phone.
8. Synced the phone.
9. Playlists were added AND they had the proper artwork
10. Satisfied, I checked all the playlists (meaning, I did not check "sync my whole library")
11. Clicked sync.

And then I waited...

And that did the trick. Like magic!

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Rationale: I reasoned that for some bizarre reason, the iPhone was pulling images to build playlists from the songs, and so somewhere that process was garbled. The albums and songs had the proper images, but the playlists did not. So I figured "resetting" those relationships might work. So, I experimented by deleting all the songs and albums of a single playlist, and then reloading from scratch. My guess is that this "reset" the song >> album >> playlist image association.

My reasoning might have been wrong, but hey, I can't complain about the results -- 13,000 + songs all look perfectly lovely now with all their correct images, and I did not have to reload images from any source.