解析Facebook封面图片JSON文件

I am using Facebook Graph API to get the user's cover picture. The information you get with the link https://graph.facebook.com/facebookuserid?fields=cover is a JSON object, where the link to the picture is included. How do I obtain that link with JQuery or JavaScript?

This is what I've tried:

HTML:

<div id="cover"></div>

JQuery:

$(document).ready(function(){

var URL = 'https://graph.facebook.com/<?= $user ?>?fields=cover';

    $.ajax({ 
    url : URL, 
    dataType : "jsonp", 
    success : function(parsed_json) { 
        var coverpic = parsed_json["cover"]["source"];
        $("#cover").append('<img src="' + coverpic + '"/>");
    }
});
});

Where $user is the PHP code $user = $facebook->getUser(); that gives me the ID of the current user.

Thanks.

$.ajax({ 
   url : URL,
   type: "GET",
   dataType : "jsonp", 
   success : function (parsed_json) { 
      var coverpic = parsed_json["cover"]["source"];
      $("#cover").append('<img src="' + coverpic + '">');
   }
});

here is a demo: http://jsfiddle.net/nn007/rMV5b/