Author here, hi all. There are a number of ways to skin this cat. This is my preferred method. What's yours?
https://developers.google.com/youtube/v3/docs/thumbnails
Maxresdefault is overkill for this purpose IMO. Why waste the data? Incidentally this is why I run an add-on that redirects all youtube thumbnails to mqdefault, it saves me a nice chunk of bandwidth / memory / perf.