Classified Plugin: Contact Form Captcha Not Working

URL:http://jeffspiratescove.com/classified/test-item/

When I 'Contact Seller' the contact form that opens does not have captcha image so I cannot use the contact form. If I open the URL of the broken image, I get this error:

/plugins/classifieds/ui-front/general/cf-captcha-image.php

Fatal error: Call to undefined function imagettfbbox() in /home/jpc/public_html/wp-content/plugins/classifieds/ui-front/general/cf-captcha-image.php on line 6