Each video clip on YouTube has a thumbnail image. They make it a little clearer what’s included within the videos. Although there’s only one thumbnail image for each video, you can view more thumbnails with the YouTube Video extension.
How to display more thumbnail images in YouTube videos in Chrome
The YouTube Video extension is one that shows you multiple thumbnail images when you hover the cursor over the videos. Open this page  and click on the + Free button to add the extension to your browser. When you open a YouTube page with a video on it, you’ll find a Video Preview icon on the address bar as below.
Open this YouTube page  to try it out. Now move the cursor over one of the video thumbnails on that page. Multiple thumbnails for the video are then shown like a slideshow. This video  provides a demonstration of the extension.
You can select further options for the extension by right-click the Video Preview icon on the address bar. Then select options from the context menu to open the window below. There you can adjust the thumbnail rotate speed by dragging the rotate bar left or right.
So with this extension you can now see many more image thumbnails before you play a YouTube video. As such, this makes it much clearer what’s included within the video before you play it.