When we want to use thumbnail image of a youtube video there is an easy method to get it. If you want take the thumbnail image of this video from youtube https://www.youtube.com/watch?v=OvVKnC6gGtg ,
here you can understand this is (OvVKnC6gGtg) the identifier for this video. So to get the thumbnail image we have to use https://img.youtube.com/vi/XXXXX/mqdefault.jpg, here XXXXX can replaced by the identifier of youtube video. So in our example the thumbnail image of this video is https://img.youtube.com/vi/OvVKnC6gGtg/mqdefault.jpg.