I have no doubt that this can be automated. I also have no doubt that I won’t be the one to do it. For your edification.
Sample: Artwork from The Wire, Season 3:

Copy the link. Paste the link into your browser.

At this page, either copy the location of the image or view it.
You now just need to take the “100x100-75.” out of the image to get a crisp, clean 600x600 uncompressed jpeg of your artwork.
This is for informational use only. Please use responsibly and do not redistribute iTunes artwork without the requisite permissions, etc etc. Be cool, y’all.