To add placeholder or alter the title, description and hide of the image captcha, do as follows

Use hook_form_FORM_ID_alter() and Form API pre_render function.

E.g:

/**
* Implements hook_form_FORM_ID_alter()
*/
function MY_MODULE_OR_THEME_form_contact_site_form_alter(&$form, &$form_state, $form_id) {
$form['#pre_render'][] = 'yourform_pre_render';
}

function yourform_pre_render($element) {
  if (isset($element['captcha']['captcha_widgets']['captcha_response']['#title'])) {
    // change title
$element['captcha']['captcha_widgets']['captcha_response']['#title'] = t('Change title name');
 // placeholder add
$element['captcha']['captcha_widgets']['captcha_response']['#attributes']['placeholder'] = $element['captcha']['captcha_widgets']['captcha_response']['#title'];
 // empty title
$element['captcha']['captcha_widgets']['captcha_response']['#title'] = '';
$element['my_captcha_element']['captcha_widgets']['captcha_response']['#description'] = "New Description";
}
return $element;
}

You can also use hook_form_alter() function: