IMPORTANT:
This plugin doens’t come with the automatic installation because you can only install it after importing the demo contents and they may not be useful for everyone.
To install this plugin go to Plugins->Install and look for Video Thumbnails or download it here: https://wordpress.org/plugins/video-thumbnails/
Usage:
Just install it, you don’t have to do anything.
Documentation: https://wordpress.org/plugins/video-thumbnails/