private function getHostForImageIdentifier($imageIdentifier) {
        $dec = ord(substr($imageIdentifier, -1));

        // If this is an old image identifier (32 character hex string),
        // maintain backwards compatibility
        if (preg_match('#^[a-f0-9]{32}$#', $imageIdentifier)) {
            $dec = hexdec($imageIdentifier[0] . $imageIdentifier[1]);
        }

        return $this->serverUrls[$dec % count($this->serverUrls)];
    }