iTunes dropping album artwork

Are you using an external HD? Also, what format are you using for your music files?

Try turning on your external HD before you boot the OS. If your drive does not have a power switch, unplug it and plug it back in again. Wait a few seconds for the HD to fully start up and then power up your computer. Once the OS is booted up, make sure you see the drive mounted in your file system and then start itunes. There is a good chance that this will fix your problem.

That's OK. The delay between when you post and when your post shows up can make things very confusing. I was in a bit of a hurry when I typed my response. The reason I suggested you try all of that, is sometimes things like external HD's don't fully boot up fast enough. So if your OS and iTunes get ahead of your HD it can cause all kinds of problems. Missing artwork is a very common result when this happens.
Some of the other posts talk about tagging issues. They're absolutely correct. That can be your problem, as well.