Thank you for using Apple Support Communities. It's my understanding that since updating iTunes, some of the album artwork is now missing. I have a ton of music in my iTunes library, I definitely understand the importance of artwork. I'm happy to help.
Please follow the steps below:
Open iTunes on your computer. Make sure that you have the latest version.
Right-click or Control-click on the album that has artwork missing.
Click Get Album Artwork. To select more than one album, hold down the command key and click additional albums. iTunes adds the album artwork.
From the menu bar at the top of your computer screen, choose File > Library > Update iCloud Music Library.