I have a large amount of TV shows stored in iTunes but not all of them were purchased there, particularly as you can buy a Blu-Ray of many TV shows for a fraction of the price of a Standard Definition download. The only issue I've had is that I lacked artwork for each of these shows. In the past, I'd just screenshot the 200px or so image that showed up in the iTunes store and use that, but since getting an Apple TV (which displays the artwork at a much larger size) I decided I needed a better solution. After a weekend of sorting out my iTunes library, I decided to take a look at the iTunes Search API and see if I couldn't find a way of getting that artwork. They provide a 100x100px thumbnail as part of the API response, but fortunately (with some URL experimentation) I found a way to get the full 600x600px artwork.
If you're looking for artwork for your TV shows, simply type the name in below and you'll get back all of the artwork that is available.
Update: due to user requests, I've added an optional field so you can specify whether you want TV or Movie artwork - you can click the images to get larger versions (some movie posters display at 1600x2400).
Update: if empty images appear, this generally means that Apple has decided to lock the artwork so nobody can access it. This is the reason why you can't search for album artwork as (at the time of writing) music artwork is only viewable with a corresponding key which is time-locked and only issued when you purchase a track (or request the artwork via iTunes Match). I'm trying to find a way around it but with little success so far. I've noticed this issue starting to affect some movie artwork so if it doesn't show up then that will be the reason.