This is an Apple “script” which just needs to be installed in iTunes and then you can run it to download any missing album art. Freeware.
….is a small program that will fetch album art for one or more songs using Amazon’s XML interface, allow you to preview the art that was found, and add the art you want to iTunes. The program is controlled through the iTunes Script menu: once it is installed, album art can be downloaded by selecting one or more songs in iTunes and choosing “Fetch Art” from the Script menu. It doesn’t require an account to get art and works from any country.