A significant chunk of my iTunes library is comprised of CD rips and a few independent recordings from musician friends. As a result, a fair number of tracks lack the excellent artwork that graces iTunes. If you are in a similar situation, you can quickly add artwork to individual tracks or even several tracks with just a few clicks. This trick also works with video, too.
To add artwork to an individual clip, simply select the track in iTunes and use Command-i to open the inspector (or File > Get Info), which will show all the details of the track. You can change other details from this interface, but we are most interested in the "Artwork" tab as shown above. Select the Artwork tab and use the "Add" button to insert your custom artwork. You also can drag the artwork from Finder into the artwork area.
You also can add artwork to several tracks at once by selecting a handful of tracks and then clicking on File > Get Info or Command-i to open the inspector. Click "yes" if you are warned about editing multiple files and then select "Info" to view the contents of the information pane. You then can drag the image to the Artwork area to assign new art to all those tracks at once.
iTunes 11 supports adding JPEG, PNG, GIF, TIFF, and Photoshop files as artwork. The app will resize the images to proper size so you don't have to worry about sizing down the artwork before you add it to iTunes. You also can have iTunes do the artwork importing for you if the track is from a known album by a known artist. Just control-click the songs without artwork and choose "Get Album Artwork" from the pop-up menu. This automatic method pulls the art from a central database and is less reliable than the manual import described above as the song matching is not always accurate.