https://www.thewebtaylor.com/articles/how-to-get-a-youtube-videos-thumbnail-image-in-high-quality
So you need to use the thumbnail picture of a YouTube video, but the thumbnail shown on YouTube is too small and would look pixelated if enlarged at all. Well here’s a quick and easy tip to get the high quality (HQ) version of the video’s thumbnail.

Simply visit the following URL and insert the video ID:
http://img.youtube.com/vi/YouTubeID/maxresdefault.jpg
So if I wanted to get the full resolution image for this video: https://www.youtube.com/watch?v=DxwrdB7A6-I
The following link would display it:
http://img.youtube.com/vi/DxwrdB7A6-I/maxresdefault.jpg
You can also get other sizes using this method.

Default Thumbnail
http://img.youtube.com/vi/YouTubeID/default.jpg

High Quality Thumbnail
http://img.youtube.com/vi/YouTubeID/hqdefault.jpg

Medium Quality
http://img.youtube.com/vi/YouTubeID/mqdefault.jpg

Standard Definition
http://img.youtube.com/vi/YouTubeID/sddefault.jpg

Maximum Resolution
http://img.youtube.com/vi/YouTubeID/maxresdefault.jpg

Alternative
Unfortunately not all videos have the thumbnail images set, so the above method doesn’t work. In these cases you would have to resort to the YouTube Data API.

If you’re not developing an app or website that relies on pulling thumbnails from YouTube, this would be a bit overkill so you would probably have to resort to taking a good ol’ fashioned screenshot of the video in action.