_setBackgroundImage(img) {
    if (!img) {
      return Promise.reject(rejectMessages.loadImage);
    }

    return new Promise((resolve, reject) => {
      const canvas = this.getCanvas();

      canvas.setBackgroundImage(
        img,
        () => {
          const oImage = canvas.backgroundImage;

          if (oImage && oImage.getElement()) {
            resolve(oImage);
          } else {
            reject(rejectMessages.loadingImageFailed);
          }
        },
        imageOption
      );
    });
  }
Set background image
@param {?(fabric.Image|String)} img fabric.Image instance or URL of an image to set background to
@returns {Promise}
@private