Pretty sure the best you can get is whatever resolution the picture is saved as.
It's been a while since I have done it, but you should be able to turn on 'View Hidden Folders' in your iTunes, then hunt down the picture that way. Annoyingly, though, iTunes renames all your files to 4-character garbage so it can be quite a pain.
Alternatively, you can Google for a program that will dig up backups of your iTunes files for you, which should also include artwork. I've done it before but can't for the life of me remember the program I used.
I hope that helped in some way. D: