public function getImageUrl($imageIdentifier) {
        if (empty($imageIdentifier)) {
            throw new InvalidArgumentException('Missing image identifier');
        }

        $url = sprintf(
            $this->getHostForImageIdentifier($imageIdentifier) . '/users/%s/images/%s',
            $this->getUser(),
            $imageIdentifier
        );

        return Http\ImageUrl::factory(
            $url,
            $this->getConfig('privateKey'),
            $this->getPublicKey()
        );
    }