When you add new movies and TV shows to iTunes, it automatically creates an image to display as that video's icon. To create this image it just takes one of the first few frames of the movie, hence why it's called the poster frame. Unfortunately, this normally just gives you an image of some movie studio logo, which isn’t very descriptive.

This isn't a problem if you add a custom poster (check out GetVideoArtwork), but occasionally a proper poster isn't readily available.

A simple solution to this problem is to choose another frame of the movie to use as the poster frame. To do this, just start the video playing in iTunes, go to the frame you want to use and then pause it. Then just right-click on the video and choose "Set Poster Frame." A good frame to use is when the title of the movie or TV show is displayed.