Hover a thumbnail at its bottom part and move the mouse horizontally to view the actual screenshots from the video
As of 2019-09-29 07:55:23 UTC. See the latest version.
Works on www.youtube.com in any page with video thumbnails.
Displays real screenshots from the videos (aka "storyboard").
Some videos don't have storyboards.
This is how YouTube serves them.
Tested in Chrome + Tampermonkey.