def _pil_props(self):
        """
        A tuple containing useful image properties extracted from this image
        using Pillow (Python Imaging Library, or 'PIL').
        """
        stream = BytesIO(self._blob)
        pil_image = PIL_Image.open(stream)
        format = pil_image.format
        width_px, height_px = pil_image.size
        dpi = pil_image.info.get('dpi')
        stream.close()
        return (format, (width_px, height_px), dpi)
A tuple containing useful image properties extracted from this image
        using Pillow (Python Imaging Library, or 'PIL').