In addition to the GD Library, the True type library must also be enabled.
trying to access the file that generates the captcha image:
WAVT_CaptchaSecurityImages.php
shows a blank screen.
this means that there is a php error occurring but error reporting is turned off.
add the following code to line 1 of the that page to turn error reporting on:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
?>