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

Quick Tip: How To Get YouTube Thumbnail Images

3rd Thumbnail Image, Small (120×90)

http://img.youtube.com/vi/bQVoAWSP7k4/3.jpg

Quick Tip: How To Get YouTube Thumbnail Images

2nd | Default Thumbnail Image, Small (120×90)

http://img.youtube.com/vi/bQVoAWSP7k4/2.jpg

Quick Tip: How To Get YouTube Thumbnail Images

Default Thumbnail Image, Full-Size (480×360)

http://img.youtube.com/vi/bQVoAWSP7k4/0.jpg

Quick Tip: How To Get YouTube Thumbnail Images

Best regards