I had a similar problem. My artwork on some of songs disappeared only on my iPod not iTunes. Also, the names of the songs stopped being centered, they became aligned to the left.
Solutions:
1. I've manually deleted the old artwork and pasted new art and sometimes used the download "missing album artwork." It worked but it was a time consuming pain.
2. I googled to find a faster solution. Found this thread. It didn't really help. I then found this site
http://www.downloadsquad.com/2006/11/30/how-to-fix-your-itunes-artwork/
What you should do is highlight all your songs (or which ones you want). Then Right click to get to the "edit information" window. Check on the artwork square so that all your songs will have the same blank artwork. Then click ok. All your artwork should be reset. If not, highlight everything again and right click and choose "clear downloaded artwork."
Now to get your artwork back. Highlight everything again. Right click and select "get album artwork" or go the "Advanced" tab in the top left corner.
(Sorry, I only have PC... I think you ca still do this with Macs if you know where to find the quoted parts in your iTunes.)