If you want to download thumbnails then first you go to youtube.com and select a video which has thumbnail on the video So you want youtube video link, how can you get the link of the video which has thumbnail
Click on the share button with the symbol in the given picture and copy the link Then click on the button below
After that you will go to a website then paste that YouTube link on that website where it is marked in the picture below
When you paste the link in the place mentioned in the picture, you will see a thumbnail on that link and after that as soon as you click on the download button, this thumbnail will be downloaded.
Note :- it works only on desktop you can also use it on desktop view in phone