Several users have reported that after upgrading their device to iOS 5, some or all artwork has disappeared on their iPod Touch /iPad/iPhone.
- Connect your iPod to the computer.
- Select your device, then the ‘music‘ tab
- uncheck ‘Sync Music’
- Sync your device
- recheck ‘Sync Music’
- Re-sync your device.
Please note that this process may take some time especially if you have a lot of music files.