xxxxxxxxxx
var iframe = $('iframe:first');
var iframe_src = iframe.attr('src');
var youtube_video_id = iframe_src.match(/youtube\.com.*(\?v=|\/embed\/)(.{11})/).pop();
if (youtube_video_id.length == 11) {
var video_thumbnail = $('<img src="//img.youtube.com/vi/'+youtube_video_id+'/0.jpg">');
$(body).append(video_thumbnail);
}
xxxxxxxxxx
https://img.youtube.com/vi/<insert-youtube-video-id-here>/0.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/1.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/2.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/3.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/default.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/hqdefault.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/mqdefault.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/sddefault.jpg
https://img.youtube.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg