How to use custom captchas in contact forms
...and replace the following code with your captcha, that might do it!
<div class="captcha"> <label><?php echo $translatedCaptchaLabel; ?></label> <br /> <?php $captcha->display(); ?> <br /> <?php $captcha->showInput(); ?> </div>
I'm not an expert, but I think this should do it.
Additionally this custom template will create some additional styling work for you because it takes away the tables - indicated by it's name - to allow for more flexibility it how your forms can look. All of this is mentioned in the marketplace, but I didn't want you to be taken off guard!
For example this is my fork of the concrete5 recaptcha package.
It is pretty straightforward to create your own to work with that service. Then you would just choose that captcha for your site and it would show up everywhere you had a captcha.