When you look at videos in your iTunes library, unless you’ve added artwork for movies and TV shows, it can be hard to tell them apart. It’s especially difficult for your home videos, because there is no artwork. By default, iTunes displays the first frame of the video, but you can change what is displayed by setting a poster frame.
A poster frame is a frame that’s chosen to be used to display in iTunes, and on an Apple TV. In a sense, it’s the equivalent of album artwork for music, except that you choose it from the video. I use this for videos that I buy that don’t have artwork; for example, I’ve bought a few videos from Globe On Screen, a website that sells filmed plays from Shakespeare’s Globe Theatre in London. For some of them, I can find artwork that was used to advertise the plays, or that was used when they were released on DVD. But not all of them have artwork available. The last one I bought, Gabriel, has no artwork, so I set a poster frame.
To set a poster frame, play the video, and find the shot you want to use. When you get to the shot, stop the video, drag the playhead to the exact spot you want to use, then right-click on the video.
Once this is done, you can drag the playhead back to the beginning of the video, or just close the window. When you play it, iTunes or your Apple TV may ask if you want to resume playing from the last location, or start at the beginning; choose the latter.
You’ll see the video displayed with the poster frame in iTunes, and in the Apple TV interface. As you can see here, it’s not perfect; movies usually show with vertical artwork. If you wish, you can take a screenshot of any part of your video and add it as artwork, but for most videos, a poster frame is all you need.