@KatieK I didn't downvote, but questions about the implementation details of Apple's software (in this case, how iTunes matches artwork) are usually a poor fit for StackExchange, because real information about those details is rarely available; as a result, the answers to such questions tend to be subjective / prone to conjecture. 
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Dan JMar 26 '12 at 18:26

6 Answers
6

Clearly, the database being polled either has errors or the mix of the data used to determine the track listing isn't uniquely specified which leads to the errors. 

Here is what you have in your control:

Manually override these artworks by hand or by using a competing service.  There are many and I've never found them to be worthwhile for the music I have issues with.  YMMV of course and I know several people really love these iTunes add-on programs and cleaners. 

The metadata - you can edit the title or the author or any of the other music tag information on the problematic tracks to better match what Apple sells the music.  If you have a Jimmy Buffet album labeled "stoner rock" for genre - perhaps changing it to "Rock" would be the difference in retrieving the correct artwork. 

As you can see, the details aren't public, so some trial and error may be needed.  If you have the actual CD and can re-encode them, usually this gets the data correct for matching with artwork. 

iTunes uses an external site to search for album art.  I don't remember exactly which site it is, but I do remember that it was crowdsourced, so there's a high likelihood that, especially with relatively rare music, it will get the wrong thing. 

This seems to be an ongoing problem with iTunes and MP3s.  It'll get better for awhile, then suddenly a ton of threads on it will pop up on apple's support boards.  It might improve a little, but many will still report problems. 

Even when the ID3 tags match exactly with the information for the same album in the iTunes store people indicate they are still getting mismatches or missing artwork.  Others have complained that after going through and painstakingly attaching artwork manually to all the songs that had missing artwork, a later iTunes upgrade has removed some or all of the artwork they added. 

The problem is so pervasive that there are apparently several programs intended to do the work that iTunes should be doing itself:

Yes, it could be a problem for songs not puchased from the Store.  I advise to store a covers with all full albums / compilations you have.  I do so and haven't these situations because I add the covers when I edit meta info of the files (using The Tagger utility). 

itunes just gets confused sometimes i guess, an easy solution is to go to google, copy the album art you want and go to the album you want to change in itunes.  click on advanced and add album art (top left hand corner).  then you go to album art i believe and control and copy/paste the picture into the album art space.  this allows you to get custom art work!