My artwork in iTunes for Windows was randomly disappearing. In addition to the artwork disappearing, I had new information appearing in the track info when I played a song. Sometimes it was a new composer, or a new spelling of the same composer. It turns out Windows Media Player is responsible for this. It is getting the album information via a background process, but it makes it look like iTunes is doing. How nice.
The Fix? Go into WMP options and tell it to stop going to the Internet to retrieve album information. In my version I did this on the Library and Privacy tabs.