The artwork is not on the CD itself. It is submitted online just like the track and album information. When someone puts the CD into a program like iTunes, the CD information is collected and then the software looks for a match to it in the Gracenote database and pulls the track listings. I am not as familiar with this process for album art, but I do not nothing is on the CD itself.
I know that if you sell your music on iTunes, your album artwork is uploaded as part of your music files and then hosted in their database. I am not sure how to get it “up there” other than this.
Hope that helps with something! Let us know if you find a better answer as I would be curious what the answer to your question is.
Best of luck….