MUNCHKIN  Fetch Art 1.3.11 doesn't work under Leopard. But while waiting for an update, there is a work-around:

1) Select a song or songs from the *same* album.

2) Launch Fetch Art to go fetch the album art.

3) Assuming Fetch Art finds the right artwork, click on the first song in the Fetch Art dialogue and click on the "Copy to iTunes" button. This will produce an error message.

4) Click the "OK" button on the error message, then,

5) Quit Fetch Art from the "File" menu. This sends the artwork to the trash.

6) Go to the trash and drag & drop the artwork on to the album art window in iTunes. Done!

7) Empty the trash.  
(Version 1.3.11)