I found that it is possible to make it visible again by selecting an item (say a movie), pressing Command-I, clicking on the Artwork tab, then selecting the artwork, cutting it and pasting it back. After pressing OK, the artwork displayed again. (Though for some files, I needed to quit and relaunch iTunes to see the artwork.) In some cases, for movies, simply displaying the Artwork tab of the Info window was enough to get iTunes to display the art.
With albums, it was a bit more difficult. I needed to select a single item to copy the artwork, then select an entire album to paste it. But this seems to work for all my files.
I wish there were a way to automate this. Doug, any thoughts?

