Very peculiar issue for me on my iPhone 4 and 5 on iOS 6. About a dozen of my songs are missing art when viewing through Cover Flow only. The art shows up fine in portrait mode. When playing one of the affected songs in portrait mode, then turning to landscape, it will show the art UNTIL I swipe off the song.
I have removed the songs, synced, added them back, synced, and no change. I have removed the songs, removed the art from the songs, put the art BACK on via iTunes, synced, and still no art via Cover Flow.
-HM