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

@classmethod
def image_url(cls, width=None, height=None):

    width_ = (width or cls.random_int(max=1024))
    height_ = (height or cls.random_int(max=1024))
    placeholder_url = cls.random_element(cls.image_placeholder_services)
    return placeholder_url.format(width=width_, height=height_)
