An authentication is provided with the script. The Login works for single user ONLY IF it is enabled in 'config.php'. If you already use a password protected folder, you can disable this user login. For protecting folders the simplest way is basic http authentication using '.htaccess' files. Refer your host provider for more details.
Load the gd2 library for PHP. It allows use of thumbnails. If thumbnails won't appear, either you havn't installed gd library oe it is not enabled in php.ini. Usually in Win32 platform gd2.dll comes with php installation.