Having the thumbnail images of a Youtube video is very useful for things likeblogging and social media, and is a very simple, straightforward process.
Just follow the instructions in the video, and use the following format to get the url of any youtube video image:
http://img.youtube.com/vi//0.jpg Full Size (480×360)
http://img.youtube.com/vi//1.jpg Small Size(120×90)
http://img.youtube.com/vi//2.jpg Small Size(120×90)
http://img.youtube.com/vi//3.jpg Small Size(120×90)