code: function(url) {
          var img = this.imgDiv;
          if (url) {
              img.style.visibility = 'hidden';
              img.style.opacity = 0;
              // don't set crossOrigin if the url is a data URL
              if (this.crossOriginKeyword) {
                  if (url.substr(0, 5) !== 'data:') {
                      img.setAttribute("crossorigin", this.crossOriginKeyword);
                  } else {
                      img.removeAttribute("crossorigin");
                  }
              }
              img.src = url;
          } else {
              // Remove reference to the image, and leave it to the browser's
              // caching and garbage collection.
              this.stopLoading();
              this.imgDiv = null;
              if (img.parentNode) {
                  img.parentNode.removeChild(img);
              }
          }
      }
docs: Method: setImgSrc
  Sets the source for the tile image
  
  Parameters:
  url - {String} or undefined to hide the image