That's frustrating. I find it ludicrous that anyone can find the art online, but for whatever reason, your database doesn't contain said art. How does your site get the art for the albums? Were you given just a bunch of art files for all of these albums? That doesn't seem reasonable.
Even with user submissions, surely independent verification can happen. I mean, really.
Hi James, Our album art is furnished by a company called Rovi that provides us with our music information (reviews, track lists, etc). More info is available in our FAQ: http://www.allmusic.com/faq#whoisrovi
They are only able to use album images provided by the artists, the labels, or the copyright holders in an effort to provide the correct cover.
While it would be nice to be able to accept uploaded images from anybody, we aren't currently staffed to monitor and validate the submissions as they would come flooding in.