Returns URL to placeholder image
        Example: http://placehold.it/640x480

def image_url(self, width=None, height=None):
        """
        Returns URL to placeholder image
        Example: http://placehold.it/640x480
        """
        width_ = width or self.random_int(max=1024)
        height_ = height or self.random_int(max=1024)
        placeholder_url = self.random_element(self.image_placeholder_services)
        return placeholder_url.format(width=width_, height=height_)