Since updating to cm 10.1 with android 4.2.2 my app is generating much improved album art
You may want to check your app updates because Google Play Music just got an awesome new update.
Model: Nexus 7 (2013) 32GB
OS: Android 4.4.4 KitKat
I had a song in Google Play Music. There were other songs by the same artists, and along with those I downloaded new album art for all of them. Ever single song, (including the old one) i replaced the album art on.