docs: 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);
      }