You can choose from three methods to download YouTube thumbnail:
Replace "youtube.com" with "http://downloadthumbnail.com" to download youtube thumbnail.
For example: http://www.youtube.com/watch?v=7mEeqs_zXwo
would turn to: http://downloadthumbnail.com/watch?v=vKYm794Twck
Copy the URL of the YouTube video which thumbnails you would like to download
put the URL of the YouTube video into the input and then press the "Show the thumbnail" button.
Copy the ID of the video which thumbnails would you like to download and
put the ID of the YouTube video into the input and then press the "Show the thumbnail"button.
Example: the URL of the video is http://www.youtube.com/watch?v=7mEeqs_zXwo
the video ID is 7mEeqs_zXwo;
After the page is loaded right click on the one of the nine different picture and select Save image as... so easy :)