Captcha seems not to work here. I get a broken image. When I call the image URL directly I get:

[phpBB Debug] PHP Notice: in file /includes/captcha/captcha_gd.php on line 82: imagettftext() [function.imagettftext]: any2eucjp(): invalid code in input string
[phpBB Debug] PHP Notice: in file /includes/captcha/captcha_gd.php on line 82: imagettftext() [function.imagettftext]: any2eucjp(): invalid code in input string
[phpBB Debug] PHP Notice: in file /includes/captcha/captcha_gd.php on line 82: imagettftext() [function.imagettftext]: any2eucjp(): invalid code in input string
[phpBB Debug] PHP Notice: in file /includes/captcha/captcha_gd.php on line 121: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2946)
[phpBB Debug] PHP Notice: in file /includes/captcha/captcha_gd.php on line 122: Cannot modify header information - headers already sent by (output started at /includes/functions.php:2946)

This seems to be charset-related. If I do following change to narrow down the created characters, the captcha starts to work.

--- captcha_gd.php      2006-11-28 07:55:33.000000000 +0100

+++ captcha_gd.php.w    2006-11-17 20:12:53.000000000 +0100

@@ -78,7 +78,7 @@

                                $x              = mt_rand(0, 360);

                                $y              = mt_rand(0, (int)($this->height - ($size / 5)));

                                $color  = $func2($image, mt_rand(160, 224), mt_rand(160, 224), mt_rand(160, 224));

-                               $text   = chr(mt_rand(45, 250));

+                               $text   = chr(mt_rand(45, 127));

                                imagettftext($image, $size, $angle, $x, $y, $color, $this->get_font(), $text);

                        }

                }