function getScreen( url, size )
{
  if(url === null){ return ""; }

  size = (size === null) ? "big" : size;
  var vid;
  var results;

  results = url.match("[\\?&]v=([^&#]*)");

  vid = ( results === null ) ? url : results[1];

  if(size == "small"){
    return "http://img.youtube.com/vi/"+vid+"/2.jpg";
  }else {
    return "http://img.youtube.com/vi/"+vid+"/0.jpg";
  }
}

http://jquery-howto.blogspot.com/2009/02/how-to-get-youtube-video-screenshot.html

Lets you get YouTube video scene images by video id or youtube video url using only javascript OR jQuery.

Get YouTube video thumbnails URL with jQuery

javascript, jquery, api

jQuery