wpmem_captcha

/**
 * The wpmem_captcha filter allows you to filter the HTML
 * for any of the available captcha methods in the plugin.
 * It filters the entire HTML string that applies the
 * captcha.
 * 
 * NOTE: This filters the pre-submitted form HTML for the
 * captcha (which is essentially the necessary JavaScript).
 * It does NOT affect anything on the form validation side
 * of things, so you cannot use this alone to change the
 * capctha type.  That is a more involved process. You
 * can, however, use this to customize the HTML of the 
 * existing captcha type you are using (which is what it
 * is intended for).
 */
add_filter( 'wpmem_captcha', function( $captcha_html ) {

    // your custom HTML for the captcha here.

    return $captcha_html;
});