When i try to view the file that creates the captcha image directly at:
WAVT_CaptchaSecurityImages.php
I get a 500 server error, This is a generic error message that really doesn't give enough information to troubleshoot.
Most likely the generic error is being displayed because has configured the server not to show php errors.
Please contact the host to have them either turn on more robust errror reporting or look into the servers error logs to see what the real error message is.