In the good old days, you could drag it to the artwork box that seems to no longer exist. I've tried Get Info and then copy & paste to the artwork box - it doesn't take. Also, the artwork tab will be either non-existent or grayed out. Any other ideas?
iTunes 11.4-OTHER, Mac OS X (10.6.8)
First, your profile (last line in your post above) indicates an old version of iTunes, and an old version of OS X. So I don't know if what I'm about to suggest existed when these versions were current.
Second, graying out that you observe may indicate an ownership issue. Perhaps your login ID does not own the files, or your Apple ID does not own rights to the music.
But if all were well and all were current, you would see an "Add Artwork" button at the bottom once you did a "Get Info" and looked on the artwork tab:
How does one add artwork manually?
