Getting a YouTube thumbnail is actually quite simple. All you need is the video ID and the corresponding resolution URL. It may sound complicated, but take a look at the image below and you will immediately understand how YouTube thumbnail downloading works.

As shown above, different thumbnail resolutions correspond to different URLs. For example, to get the MQ thumbnail of a video, you just need to insert the YouTube video ID into this URL: https://i1.ytimg.com/vi/{VIDEO ID}/mqdefault.jpg.
So if the video ID is '7na_-LfGfpE', the MQ thumbnail URL would be: https://i1.ytimg.com/vi/7na_-LfGfpE/mqdefault.jpg.
How do you find the video ID? YouTube URLs come in two formats: https://www.youtube.com/watch?v={Video ID} and https://youtu.be/{Video ID}. The video ID is always an 11-character string of letters, numbers, and symbols.
This YouTube thumbnail downloader saves you the trouble of manually constructing these URLs.