Get YouTube Thumbnail image in 3 clicks (in 2016)
Ever since YouTube has removed the Stop feature from its player, people need another trick to view the video thumbnail that most of the time is the call to click from the related videos list.
How to see a YouTube video's thumbnail image in full resolution?
EASIEST: go to YouTube Thumbnail Viewer and Downloader
But to do it manually:
1. Go to your desired video. Pause the video so that autoplay won't accidentally forward to the next one.
2. Click CTRL + U to view the source code of the video page.
3. Click CTRL + F to find og:image (should be around row 55)
4. Copy the url from the content="" and paste it to another browser tab/window.
5. Voila! You have just opened the video thumbnail of the video. Probably it looks something like this. Right click on this image and select "Open in new tab" to see it's url. Should be like: https://i.ytimg.com/vi/4oNhSeTSrQg/maxresdefault.jpg
This is the thumbnail image for this mix. Click on Play and you'll see that there is a totally different image as the video background. So hopefully this little trick will help you find the thumbnail images of the videos you need.
Note: the image above doesn't belong to me and is displayed only for informational purposes from a music mix from DJEkkiMusic's channel. Feel free to subscribe to him for more.