Movie Covers allows you to update your cover artwork in iTunes. When you start Movie Covers, your movie titles will be listed to the left of the window. If you have a particularly large collection of movies, the search field at the top-left of the window can be used to filter your list.
By clicking on one of your movie titles in the left-most list, the middle list will, after a brief delay, show a list of movie titles that may hold the movie cover that you would like to appear on your movie in iTunes. You just pick a result and click the "Update Movie Cover in iTunes" button at the middle bottom of the window.
Repeat this procedure for every movie in your collection for a vastly superior Apple TV, iPod, and iPad experience! Oh, and try resizing the window if you would like to see a larger version of the preview.