Adding fields via ajax in form does not show in arrays in form_submit
The form builder function is what Drupal refers to whenever it needs to rebuild the form. Parts of the form should not be built in the ajax callback. Instead, you can return them using something like

function add_img_upload($form,&amp;$form_state) {
  return $form['img_mng'];
}


Your form builder has access to form state information, which allows you to build the form conditionally depending on the state of the form.