getLargestYoutubeThumbnail - Because maxresdefault.jpg isn't reliable. Requires jQuery/Zepto, example here: http://jsbin.com/bamiju/edit?html,css,js,output

getLargestYoutubeThumbnail.js

// getLargestYoutubeThumbnail by @ReallyGoodTeam, MIT

/* Sample youtube IDs

teYZM0Aft0A
wr1fa3ZaNYY
8e-BsJoaS20 - doesn't have maxresdefault

*/

function getLargestYoutubeThumbnail(youtubeId) {
  var def = $.Deferred();
  var img = new Image();
  img.onload = function() {
    if (this.width < 720) {
      def.resolve(img.src.replace('maxres', 'hq'));
    } else {
      def.resolve(img.src)
    }
  };
  img.src = 'https://img.youtube.com/vi/' + youtubeId + '/maxresdefault.jpg'
  
  return def.promise();
}