Man... I just typed up a fairly nice step by step for this and got shut down by the "hey you already were logged in but you should log in again and lose your long wordy post" bridge... So here are the basics of pulling this off basically perfectly.
Find the actual iTunes store page for your content (movie or TV Show).
Right click on the image you want and select copy link.
Open your browser (Safari in my case) and paste in your copied link and go
It will take you back to the iTunes store page you just came from, but that's OK
Go back to your browser and notice that the artwork you're looking for in right there on the page
Right click on the image and copy it
Go to your ripped content and select whatever you want to have this particular artwork, go to get info and paste your copied image into the artwork space
Tada... "Official" artwork for your ripped/backed-up video content.
Sorry for bumping a two year old post, but I know since I was searching for it now, someone will again, and this can be helpful
Cheers!