If you log in to your iTunes Store account you can have iTunes get album artwork for your missing albums provided they are in the iTunes Store. Just mark all within iTunes and right-click to select Get Album Artwork.
I found this app http://www.tune-instructor.de/com/features.html - I have no personal experience with this so I can't guarantee anything will work in your favour. I just remembered that the app was available. It's free for personal use.