Add CAPTCHA to guest posting on phpBB message boards

Need someone to install or build a CAPTCHA for guest posters on phpbb message boards. SEE EXAMPLE (ATTACHED). The captcha should apply to GUEST POSTING ONLY. In other words, registered users should not be presented with the CAPTCHA challenge. Only unregistered guest posters. We need the code and instructions for reinstalling in the future. Ideally, the CAPTCHA should be strong, and flexible enough to modify and tweak, should spammers again circumvent the CAPTCHA. This is listed as a "very small" project ($30-100) but will consider higher bids (for custom builds only). In your reply, please let me know if you plan to install or build. Responses without these details will be ignored!