The captcha script must be included first thing in the page that contains your form.
Adjust your form so that the captcha script processes it instead of your own form handler.
Add the captcha image and input box to your form.<img src="/captcha/image.php" alt="captcha" title="captcha" width="210" height="50" /><br />
You may alter the above code to better match the style and layout of your site, as long as the input element name remains unchanged.
Customise the captcha_error.php file to match your own site. You may leave it simply with a message advising that the captcha validation failed, or you might like to put a duplicate of your original form in it to save the user from having to press 'go back'.
If you want to do this, you can save them having to re-enter their information in the form by setting the value attribute in the input fields using the POST data from PHP: