Each YouTube video has four generated images from the youtube video known as thumbnails.
For example, if your video has this URL:
1 | https://www.youtube.com/watch?v=YOUR_VIDEO |
You can see and change the URL to see this thumbnails:
1 2 3 4 | https://img.youtube.com/vi/YOUR_VIDEO/0.jpg https://img.youtube.com/vi/YOUR_VIDEO/1.jpg https://img.youtube.com/vi/YOUR_VIDEO/2.jpg https://img.youtube.com/vi/YOUR_VIDEO/3.jpg |