1. Open the youtube video link in a browser

2. Right click (on the white space) => View the page source. Or for example in Google Chrome, Tools => View Source

3. Find (Ctrl+F) => “og:image”

4. Copy the og:image content url. For Example, “https://i1.ytimg.com/vi/M0XTPSYdP08/hqdefault.jpg”

5. Open the copied og:image content url in the same browser window

6. Right click on the image and save it to your desktop

https://clients.javapipe.com/knowledgebase/101/How-to-Download-Thumbnail-or-Cover-Images-of-YouTube-Videos.html