How to set the thumbnail image on HTML5 video?

Display Your Video First Frame as Thumbnail:

Add preload="metadata" to your video tag and the second of the first frame #t=0.5 to your video source:

<video width="400" controls="controls" preload="metadata">
  <source src="https://www.w3schools.com/html/mov_bbb.mp4#t=0.5" type="video/mp4">
</video>

Add poster="placeholder.png" to the video tag.

<video width="470" height="255" poster="placeholder.png" controls>
    <source src="video.mp4" type="video/mp4">
    <source src="video.ogg" type="video/ogg">
    <source src="video.webm" type="video/webm">
    <object data="video.mp4" width="470" height="255">
    <embed src="video.swf" width="470" height="255">
    </object>
</video>

Does that work?