Prepare watermark image
        :return: Image.Image

def _get_image(self):
        """
        Prepare watermark image
        :return: Image.Image
        """
        if self.image is None:
            image_path = '%s/%s' % (current_app.static_folder, os.path.normpath(self.image_path))
            try:
                self.image = Image.open(image_path)
                self._reduce_opacity()
            except Exception as err:
                raise ValueError('Unsupported watermark format: %s' % str(err))

        return self.image