Replace VIDEO_ID with the YouTube video's ID.
Example:
If the video URL is: CBX-6 Club of Australia National Rally 2018 in Jindabyne, NSW
The video ID is:
0u9c1zAOrQY Therefore the thumbnail URL becomes: Open Full Resolution Thumbnail
You can obtain the video ID from the address bar. In this example:
https://www.youtube.com/watch?v=0u9c1zAOrQY everything after v= is the video ID.i.ytimg.com/vi/VIDEO_ID/maxresdefault.jpg