Get scaled watermark image
        :param resource: Image.Image
        :return: Image.Image

def _get_scaled_image(self, resource):
        """
        Get scaled watermark image
        :param resource: Image.Image
        :return: Image.Image
        """
        image = self._get_image()
        original_width, original_height = resource.size

        k = image.size[0] / float(image.size[1])

        if image.size[0] >= image.size[1]:
            target_width = int(original_width * self.size)
            target_height = int(target_width / k)
        else:
            target_height = int(original_height * self.size)
            target_width = int(target_height * k)

        image = image.resize((target_width, target_height), Image.ANTIALIAS)

        return image