iTunes being a bitch
If you’ve had album artwork in iTunes that’s disappeared, even though you know it was there previously (possibly due to iCloud or iTunes Match), try this fix: go into the album, right-click the first track on the album and go to Get Info (or just hit Cmd-I). Either the missing album art will reappear instantly and you can simply cancel out, or you’ll have to click Next to go to the next track on the album and then it will reappear. Rinse and repeat for every album missing its previously stored artwork. It worked for every last album (as soon as I noticed it, that is—after I’d already wasted a bunch of time finding new artwork).