How to restore missing iTunes artwork
For some reason, iTunes sometimes has problems displaying the artwork for albums, movies and TV shows. One day it will display fine, the next it won't. Here are some tricks you can try to restore any missing artwork.
How successful these tricks are at restoring the missing artwork seems to vary from user to user so your mileage may vary, however in most cases at least one method should work.
Restore missing artwork by viewing song info
In iTunes, navigate to the album, movie or TV show that is missing its artwork. If its an album or TV series right click on the first song or episode in the list, for movies simply right-click on the movie itself. Then click on Get Info. A properties box will popup with information about the selected media.
The actual contents of the properties box may vary depending on exactly what type of media you selected.
You should see the artwork in the top right of the properties box. If so, click OK to close the properties box and the missing artwork should be restored in iTunes' main view. You can view a larger version of the artwork by clicking on the Artwork tab within the properties box.
More often than not, this will restore any missing artwork. Sometimes, however, the artwork will not display even in the properties box. In those cases, you should try one of the alternative methods listed below.
Restore missing artwork by clearing the iTunes cache
For performance reasons, iTunes stores a cache of all the artwork it downloads in a folder on your Mac. Occasionally it seems the cache can become corrupted which causes artwork to not display. Deleting the cache forces iTunes to re-download all the artwork and often restores the missing artwork.
To do this, quit iTunes (if you have it open) and then open Finder and navigate to the Music folder in your user home directory. Then click into iTunes -> Album Artwork. There should be a folder named Cache. Drag it to your trash to delete it. Upon re-opening iTunes it will recreate the Cache folder and re-download all your artwork, in the process hopefully also restoring any that was missing.
Restore missing artwork by updating iTunes
As a last resort if none of the above methods work, its always worth checking to see if an update to iTunes is available. Updating iTunes clears out old caches and files which can help restore missing artwork.
If you're using a recent version of OS X, the chances are iTunes is already up to date thanks to Automatic Updates. To check, click on the Apple icon in the OS X menu bar and choose App Store. If updates to iTunes (or any other programs you've installed through the App Store) are available, they'll be listed under the Updates tab.