I restored my iPhone 5 from an iOS 7.0.6 jailbreak to iOS 7.1. I restored to a backup. During the syncing of songs from the backup to my iPhone it accidentally got unplugged. I plugged it back in and synced my songs manually (not from the backup).
Now 5 songs that I bought from iTunes have all lost their album cover on my iPhone. All the other album covers from other songs are still there. It's not a big deal, but I like to have all album covers filled in.
While I was typing this I remembered that it helped to completely delete the album covers and download them again, I did this, now only 2 albums are naked, so to speak.