YouTube offers images that don’t have the 4:3 ratio black strips. To get a 16:9 video thumbnail with no black strips, try one of these:
http://img.youtube.com/vi/<insert-youtube-video-id-here>/mqdefault.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg
The first one mqdefault comes as a 320×180 pixel image.
The second one maxresdefault comes as a 1500×900 pixel image, so it would need resizing to use as a thumbnail. This is a nice image but it isn’t always available. If the video is of a low quality (less than 720p I’d imagine, not exactly sure) then this ‘maxresdefault’ becomes unavailable. So never rely on it.