I have a number of movies and TV shows in my iTunes library, and they are not all from the iTunes Store. However, I like to have my library properly tagged and organized, so I manually add artwork to my movies and TV shows. Most of the time when I do this, everything works as expected. However, sometimes when in Grid view, I don't see the artwork for certain items, yet I do when I use Get Info or look at the Selected Item panel under the playlists.
It turns out the solution is simple. If you add artwork and it doesn't show up, simply remove the added item from iTunes and re-add it. This will pick up the artwork and your library will remain organized.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090807170931340