Maybe:
http://www.mcelhearn.com/deleting-the-itunes-album-artwork-cache-folder-may-reso lve-problems-with-album-art-displaying/
If not then:
Try:
- Reset the iOS device. Nothing will be lost
Reset iOS device: Hold down the On/Off button and the Home button at the same time for at
least ten seconds, until the Apple logo appears.
- Unsync/delete all music and resync
To delete all music go to Settings>General>Usage>Storage>Manage Storage>Music>Tap edit in upper right and then tap the minus sign by All Music
- Reset all settings
Go to Settings > General > Reset and tap Reset All Settings.
All your preferences and settings are reset. Information (such as contacts and calendars) and media (such as songs and videos) aren’t affected.
- Restore from backup. See:
iOS: Back up and restore your iOS device with iCloud or iTunes
- Restore to factory settings/new iOS device.