How to download thumbnail images from any video on YouTube, including the auto-generated ones, the low quality ones and the high quality one. Why would you want to download a thumbnail? You might want to use it for a feature image for any theme or web application, for your post or for archive purposes or for any number of reasons.
If you have the URL of the video, you have the video ID. For example, if you have the URL of the video
https://www.youtube.com/watch?v=nnKFVh8SQ9E, the video ID is
nnKFVh8SQ9E. Now, you can use it in the following URLs:
Small YouTube-generated thumbnail #1 (120×90)
Small YouTube-generated thumbnail #2 (120×90)
Small YouTube-generated thumbnail #3 (120×90)
Small author-selected thumbnail (320×180)
Medium author-selected thumbnail at 4:3 ratio (480×360)
Large author-selected thumbnail (1280×720 or 1920×1080)*
* Only available if video has 720p or 1080p option.