code: function() {
          if (this.layer.async) {
              // Asynchronous image requests call the asynchronous getURL method
              // on the layer to fetch an image that covers 'this.bounds'.
              var id = this.asyncRequestId = (this.asyncRequestId || 0) + 1;
              this.layer.getURLasync(this.bounds, function(url) {
                  if (id == this.asyncRequestId) {
                      this.url = url;
                      this.initImage();
                  }
              }, this);
          } else {
              // synchronous image requests get the url immediately.
              this.url = this.layer.getURL(this.bounds);
              this.initImage();
          }
      }
docs: Method: renderTile
  Internal function to actually initialize the image tile,
  position it correctly, and set its url.