to enable the captcha test, you have to:

1. activate it in your config.inc.php: set $post_captcha=true

2. make sure, your PHP has the needed extentions enabled:
--with-gd
--enable-gd-native-ttf
--with-png
--with-zlib-dir
--with-ttf
--with-jpeg-dir
--with-freetype-dir
--with-xpm-dir

