Theres an app called Cover downloader from the app store you could use to search for missing covers or replace others - or you could try going to Settings - Apps - Swipe to all look for Walkman, opening it then tapping on Clear cache NOT Clear data
Another thing, I've edited some of the song titles before I upgraded my phone to 4.3 and they all went back to what they were before I edited them. Now I have to do it again. Such a pain in the a**
Possibly due to them not having a permanent tag - I edit my mp3's on my Pc using mp3tag which is free it can embed the cover and song details like artist - Tack name, year of release etc then I transfer back and don't have to worry about editing
