I’ve gotten a number of emails and tweets about the fact that iTunes 11 no longer displays album art at the bottom-left of the program’s window, and that there’s no way to easily add album art to multiple tracks. Well, this is incorrect; there has always been another way.
Select the tracks you want to add album art to: multiple tracks or an entire album. Press Command-I (or Control-I on Windows), and you’ll see the Multiple Item Information window. Drag your artwork to the Artwork well, or if you’ve copied it, click in that well and paste it. Click OK, and the artwork is applied to all the tracks.