To get the YouTube thumbnail image, you would either use their API or a simpler method would be to use this format of the url:
http://img.youtube.com/vi/VIDEO_ID/#.jpg
Where VIDEO_ID is the last bunch of characters after “v=” in the URL: http://www.youtube.com/watch?v=0TflnHlQMt8
# is the thumbnail number. The choices are 0, 1, 2, 3. 0 is the default full size image (480×360). 2 is the default small image (120×90). 1 and 3 are other optional small images that is not the default thumbnail.
source: http://www.reelseo.com/youtube-thumbnail-image/
search terms used: youtube get the image