User blog comment:ZakaSonza/How to get High-Resolution thumbnails from YouTube videos/@comment-25867278-20151202070934

The code in your example  https://i.ytimg.com/vi/MXBVGazDPQw/mqdefault.jpg is actually the ID of the video so you only have to change that part with the video ID:

https://i.ytimg.com/vi/ YouTube_video_ID_here/maxresdefault.jpg