Youtube doesn’t give an option to download video thumbnail, but that doesn’t necessarily mean you can’t do that.
First, you’ll need a link to the thumbnail. The easiest way to get it is to open the channel, locate the required video, right-click on the video thumbnail and copy the link to it.
The most important part of this link is the file name. It can be:
hqdefault.jpg
mqdefault.jpg
default.jpg
That is high, middle and standard quality. So, to get the best quality thumbnail, change the required letter in the link and download the file.