code: function (url, callback, options) {
        return fabric.util.loadImage(url, function(img) {
          this.backgroundImage = img;
          if (options && ('backgroundImageOpacity' in options)) {
              this.backgroundImageOpacity = options.backgroundImageOpacity;
          }
          if (options && ('backgroundImageStretch' in options)) {
              this.backgroundImageStretch = options.backgroundImageStretch;
          }
          callback && callback();
        }, this);
      }
docs: Sets background image for this canvas
  @method setBackgroundImage
  @param {String} url url of an image to set background to
  @param {Function} callback callback to invoke when image is loaded and set as background
  @param {Object} options optional options to set for the background image
  @return {fabric.Canvas} thisArg
  @chainable