If you want to fill out your iTunes4 library with cover art, the easy way, just do this ... go to allmusic.com 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.
[robg adds: There are a number of ways to get cover art in; another hint sumbitter suggested using the iTunes store itself, and then the capture region keystrokes (shift-command and 4) to grab the album cover area to a file, then drag that file into the cover art well.
In my own testing, I found using either allmusic or Amazon to be the easiest way to get the cover art, as you can drag and drop. The only thing I do differently is that I use the album browser to select the album I'm interested in first, then do a Get Info on the album, and drag the artwork into the small artwork box on the Get Info window. Make sure the box next to the area is checked, and then close the box. iTunes will ask if you want to modify artwork for all selected tracks; select Yes and the whole album gets the cover (artwork is otherwise assigned ONLY to the currently selected track).
You can also select multiple tracks in the track list area, but you'd probably have to re-sort your window by album to group them together first.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030429003250559