How To Get YouTube Thumbnail Images
Here is the YouTube URL format that you will need in order to view and download a thumbnail from a YouTube video:
http://img.youtube.com/vi/VIDEO_ID/#.jpg
Example :
http://img.youtube.com/vi/bQVoAWSP7k4/1.jpg
3rd Thumbnail Image, Small (120×90)
http://img.youtube.com/vi/bQVoAWSP7k4/3.jpg
2nd | Default Thumbnail Image, Small (120×90)
http://img.youtube.com/vi/bQVoAWSP7k4/2.jpg
Default Thumbnail Image, Full-Size (480×360)
http://img.youtube.com/vi/bQVoAWSP7k4/0.jpg