I use this to have a video player on browser
Using the poster attribute is the easiest way to go. Getting a preview image of the video from a time other than the start is exactly what its designed for.
http://www.w3schools.com/tags/att_video_poster.asp
Trying to create a function to dynamically grab another segment of the video to use as the poster will undoubtedly create more latency and overhead for the client, negatively affecting the UX.