function MYMODULENAME_form_alter(&$form, &$form_state, $form_id) {
  if($form_id == 'page_node_form') {
    //Adds a class to 'my image' field in the node edit form of content-type page.
    //print_r($form);
    $form['field_my_image']['#prefix']=''; //div class="myclass_name"
    $form['field_my_image']['#suffix']='';//Close div here
    //OR
    $form['field_my_image']['#attributes']['class'][]='myclass_name';
  }
}