August 30, 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 available from the iTunes Store, regardless of whether you bought the album from the Store or ripped it from your CD. 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 add your own artwork choices via the Info Panel. Select a song in an album, use Get Info (right click, cntrl-click, or via the File menu), and click the Artwork tab. You can drag images directly into the artwork well, or use the Add button to look for eligible images on your computer. More, you can add more than one image to the well and drag images around within it. When you view your library using grid or cover flow mode or have Show Artwork turned on, 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. (When you use Cover Flow to view your library, the Full Screen switch on the lower right toggles to a full screen display of the art for the album being played.)