October 25, 2010
When you're online and you use Get Album Artwork in the Advanced menu, iTunes fetches any album cover art you're missing for albums currently available in the iTunes Store, regardless of where you got them. And you can have it do that automatically when you add music to your library by checking "Automatically download missing album artwork" under the Store tab in the iTunes Preferences panel. You can also customize the art iTunes will display by adding your own artwork choices via the Info Panel.
Select a song in an album, choose Get Info from the File menu, then click the Artwork tab. You can drag images directly into the artwork well, or click Add to look for eligible images on your computer. If you add multiple images to the well you can then drag them around within it. When you view your library using any view other than List, iTunes will display the leftmost image in the well (topmost, leftmost if you've dragged in several images). This lets you use alternate images for special situations -- displaying photos of friends who particularly like the albums playing, for instance -- then switch back to the standard album art with a simple drag. To show off your custom work, choose Cover Flow and use the Full Screen switch on the lower right to toggle a full-screen display of the art for the album being played.