code: function load (url, callback, sendCallbackWhenAlreadyLoaded) {
          if (sendCallbackWhenAlreadyLoaded == undefined) {
              sendCallbackWhenAlreadyLoaded = true;
          }
  
          if (isLoaded(url)) {
              if (sendCallbackWhenAlreadyLoaded) {
                  callback(url);
              }
              return;
          }
  
          if (isLoading(url) && !sendCallbackWhenAlreadyLoaded) {
              return;
          }
  
          var c = callbacks[url];
          if (!c) {
              var image = new Image();
              image.src = url;
  
              c = [];
              callbacks[url] = c;
  
              image.onload = function (event) {
                  urls[url] = true;
                  delete callbacks[url];
  
                  for (var i = 0; i < c.length; i++) {
                      c[i](url);
                  }
              }
          }
  
          if (c.indexOf(callback) == -1) {
              c.push(callback);
          }
      }
docs: Load given image url
  @param {String} url
  @param {function} callback
  @param {boolean} sendCallbackWhenAlreadyLoaded  optional