How to change the Preview Image/Artwork of an iTunes Video

The artwork or poster frame or default image (whatever you call it) of an iTunes video (especially if its made and published from iMovie) can be changed by the following steps

1. Double click and play the video in new window
2. Pause/Stop and drag the time scale to the frame you want to set as poster frame
3. Right click on the video and select "Set Poster Frame"