code: def image_url(self, width=None, height=None):
          
          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_)
docs: Returns URL to placeholder image
          Example: http://placehold.it/640x480