MacOSXHints posted cool hints to add cover art to your existing iTunes tracks:
If you want to fill out your iTunes4 library with cover art, the easy way, just do this ... go to allmusic.com (or Amazon NdL) and find the album. Just use the Search feature there to find and navigate to your album. Then just drag the image of the cover art directly into itunes4. Be sure to pop open your cover art 'pane' first (the fourth button at the lower left of the window). Voila! Cover art is imported.
Another easy way would be to use the small app named Clutter by Sprote Research at Clutter. As iTunes plays the music, Clutter looks automatically for the cover on amazon.com and saves it on your HD ~home/Library/Images/com.sprote/clutter/. After that, all you have to do is click'n drag in iTunes 4.