the page that generates the captcha image is here:
that page shows the following error:
Fatal error: Call to undefined function imagettfbbox() in /home/exempbee/public_html/webassist/captcha/wavt_captchasecurityimages.php on line 133
the imagettfbbox() function is defined by the GD Library.
you will need to contact your host to have them enable the GD Library on the server.