code: function (url, callback) { // TODO (kangax): test callback
        fabric.util.loadImage(url, function(img) {
          this.overlayImage = img;
          callback && callback();
        }, this);
        return this;
      }
docs: Sets overlay image for this canvas
  @method setOverlayImage
  @param {String} url url of an image to set overlay to
  @param {Function} callback callback to invoke when image is loaded and set as an overlay
  @return {fabric.Canvas} thisArg
  @chainable