Thanks for the website!
So I understand that, based on the method you're using, you require exact, character for character names as they appear in iTunes?
Not really.. I'm not sure what the requirement is, but there does seem to be some leeway. For long artists/album titles for example, you can abbreviate them and it'll still find the artwork.
I tried a few purchased tracks (so I know the names match) off my recently purchased list... the results were variable. The first one came back around 900x900 (R. Kelly, Remix City Volume 1). The second at 600x600 (Alkaline Trio, Crimson). The third album I tried (Hannah Montana 2: Meet Miley Cyrus) didn't work with any of several different combinations to try to match the way it's entered in iTunes (it's a weird case...iTunes lists the artist as "Hannah Montana & Miley Cyrus" for the album, with only one each of her two names listed for any given track (the second disc is Meet Miley Cyrus). Etc.
I think the problem is the multiple artists thing.. I haven't found a way to look up compilations, etc.. I think somebody figured out a way to do it but I honestly don't remember.
Anyway, one small suggestion. If you can make it so that the artist/album fields are prepopulated with the previous attempt when there's a failure, this would make it much easier to try and change it slightly and get it to run.
Yes! Believe it or not I had that before but changed some variable names which I forgot to update. Fixed.
Depending on your 1337 coding skills, keeping track of successful album requests and/or using the search feature to identify suggestions (you can't use it to get the artwork, but you can use it to get the right listing artist/album combo) might be even better.
Not that l33t

Keeping track of album requests would likely require a mysql (or just some) database, which I've neglected learning
Haven't messed around with the search yet.
ALSO, I found a way to get thumbnails! And this works on albums even when the full sized artwork isn't available for some reason.
In addition, I might have found a way to get 600x600 artwork for the entire store. It'll take quite a bit of coding though, as of now it's using an entirely different method than I have now. (slightly more complicated for the end user.. but what can ya do?) Stay tuned
edit: Nope, scratch that ^^. The artwork that gives 404s now is still not available. It still might be worth coding though, as it lets you copy the iTunes store URL for an album and get the artwork for that, which will help solve the various-artists problem.
edit2: If anybody can figure out how to get the artwork for the artwork that ends up in 404s by modifying the URL let me know

I've updated the script to give you the unparsed URL if you want to play around with it.
ALSO
I stuck up the new method for looking up albums.. you can copy the iTunes store URL and use that instead of the search. As an added bonus, this works with albums before they're released!