iTunes suddenly will not let me add manually album art
Sure, it's a year late, but I had this problem which lead me here, so someone else might, too. I had been adding cds to iTunes and manually adding covers, then one day it would not save the cover image. Here's what I did: 

I turned off the Airplay on my iTunes.

This worked. I could save cover art again. Also changes I made to album info (fixing the album title, etc.) reverted back. Apparently, even though it showed these edits as changed, they were not saved when my speakers were set for airplay.