I will share the way "how to download the thumbnail of a youtube video". It is very simple method you need to follow a few steps to download youtube thumbnail image in high resolution.
I personally use this youtube thumbnail downloader https://thumbnailphoto.net
Here, i will share steps.
Open desired video on YouTube and copy the URL from address bar or from share button.
Paste the copied URL in text field of thumbnailphoto.net
Click on the “Download Youtube Thumbnail” Button.
After that you can download any image in any resolution by just 1 click