I have googled this. All I get from that Google search are ways to view/download the three images (0-2) that YouTube creates for a user when they upload a video. Example around that here: http://www.reelseo.com/youtube-thumbnail-image/
I was wondering if anyone knows a way to select which one of the three images to use as a poster image when embedding a video. Example: http://www.youtube.com/embed/_z-hEyVQDRA?rel=0&showinfo=0&thumbnailVersion=2