CURL: Detectar si un fichero con ruta absoluta existe

$img = "imagenes/btn-home-new-01.png";
$filename = 'https://www.midominio.com/'.$img;
$ch = curl_init();
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_URL, $filename);
curl_setopt($ch, CURLOPT_HEADER, false);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
 /*
curl_setopt($ch, CURLOPT_VERBOSE, true);
curl_setopt($ch, CURLOPT_STDERR, fopen('curl.txt', 'w'));
*/
$server_output = curl_exec($ch);
$httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
echo $httpCode."<br>";
if($httpCode == 404) {
     echo "NO";
}
else {
    echo "SI";
}
curl_close($ch);