Over the years I have been using various methods to acquire the album art for songs in my itunes collection. Now that I have permanent wifi access, getting album art for current CDs is not a problem, however I have copied many of my CDs to iTunes long before I had a permanent broadband connection. In getting album art for retrospective CD transfers I have tried various widgets and scripts. Most of which work, however most take quite some time to use as you can only go album by album through your collection. Well, here is a FAST method.
Firstly you will need to run an iTunes applescript called "tracks without artwork to playlist". This does as the name says, and creates a playlist based on tracks without artwork. Next you will need the iTunes applescript/application "Fetch Art". This allows the user to find and download the artwork to any selected tracks- despite the songs coming from one album or even various albums! This means that you can select all tracks in th e"non artwork" playlist and download their artwork in a matter of seconds.... much quicker than alternative methods. It is important to also note that all of this is free!