I was quite excited to try the new Get Album Artwork feature in iTunes 7. Unfortunately, it may not be as simple as a click. I have about 140 albums in my collection and, after the few minutes it took for iTunes to review my library and download the images, I found that there were still many gaps. Of course the music store doesn't hold every album in the world, but I found at least 20 albums that it should have grabbed the art for, but didn't.
After some further sleuthing, I discovered that the culprit may go beyond simple spelling mistakes made by myself or others (I get CD info from CDDB). It appears that inconstancies between my info and Apple's, such as genre or year of release, may make a difference.
So the tip is, if you have an album that you know is available in the store, but you can't seem to get the art to download, adjust your settings in the song's Get Info (Command-I) panel. You can compare what Apple has by selecting the album in the music store and looking to the right of the cover image. The area around the cover image in the store has most of the information you might need to adjust -- Group, Album, Genre, Copyright/Release Year.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060913232433210