A simple way to get thumbnail of a video from youtube is by using the video id:
http://img.youtube.com/vi/<your_video_id>/0.jpg
[0.jpg, 1.jpg, 2.jpg and 3.jpg can be used]
For Eg:
http://img.youtube.com/vi/-w8KI3A5zu4/0.jpg
if you got the video_id from youtube API, you can simply use this to get the image.