Cela dépends de la version que tu as

Piwigo < 2.1 -> include -> mysql.inc.php

Piwigo > 2.1 -> local -> config -> database.inc.php

le fichier est de la forme

<?php
$conf['dblayer'] = 'mysql';
$conf['db_base'] = 'aaaa';
$conf['db_user'] = 'aaaa';
$conf['db_password'] = 'bbbb';
$conf['db_host'] = 'localhost';

$prefixeTable = 'piwigo_';

define('PHPWG_INSTALLED', true);
define('PWG_CHARSET', 'utf-8');
define('DB_CHARSET', 'utf8');
define('DB_COLLATE', '');

?>

ou aaaa -> ce que tu as avant le .free.fr
et bbbb -> ton code de base MySql