the file that generates the captcha image is here:
WAVT_CaptchaSecurityImages.php
this path is returning a 500 server error.
most likely this is a permissions issue.
using FileZilla to connect to your ftp server, change the permissions for the webassist/form_validations/WAVT_CaptchaSecurityImages.php file to 755.
you can do this right clicking it in FileZilla and select the Set Permissions option.