you can also edit the webassist/captcha/wavt_captchasecurityimages.php file to be sure. add the following code at line 1 to turn on error reporting:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
?>
my bet: you will get an error about the imagettfbbox function being undefined. which would confirm the gd or freetype library being disabled.