iTunes missing artwork: the why and wherefore
It's quite common to be missing artwork for some of your iTunes music. The main reason is that iTunes only sources artwork from the iTunes store, which you must be a member of, and the store itself does not have artwork for all artists. It's frustrating, because artwork really improves the experience of browsing and listening to music.
If your iTunes artwork is missing, the first thing to try is to register for the iTunes store. This involves passing on information such as your name and address, so some people may be cautious! However, without iTunes membership, iTunes will not download album artwork.
If you're a member already, iTunes may still not have the artwork you are looking for. If an artist is not in the store, you will not see any artwork for their music. Admittedly this is rarely a problem with mainstream artists but some big name artists are absent, such as Garth Brooks, AC/DC and most tragically of all (for me anyway!) The Smiths. Until recently even The Beatles were absent!
Furthermore iTunes is unlikely to find the art for rare recordings. Some releases by artists are released by different record labels and so the artwork is not available.
The main alternative is to not rely on iTunes artwork. You can use a separate artwork application, such as bliss. Here's a walkthrough of how to find missing iTunes artwork. I hope this is useful!