If you are from any of the 22 iTunes countries, you can signup for a free account, and then get all your album art, high res, with just a click.
Here's a skitch to show you what you are supposed to do. And make sure it is showing the "Selected Track" where I've marked with the circle. And no snooping around for dirty stuff on my MacNo Luck. tried that it dosnt work. I must be doing something simply wrong. I am dragging the album artwork from say amazon to the my itunes album on the missing artwork album. I have also copied onto my desktop then tried to drag in and no joy.
There is a widget that I downloaded that works really well for finding album art. I don't have one black space on my album roll thanks to this bad boy. Here is the link:
http://www.widget-foundry.com/widgets/amazonart.htm
Hope that works for you.