Preprocess field in Display Suite module, Drupal 7 example.

mymodule.module

/*
* Before this, you have to create a "Preproces field" with name like "mycustomfield". 
* In this example we render an user image (a image field, not the user picture).
*
*/
function MYMODULE_preprocess_comment(&$vars){
  $author= user_load($vars['user']->uid);
  $settings=  array('settings' => array('image_style' => 'thumbnail'),'label' => 'hidden' );
  $vars['mycustomfield'] = render(field_view_field('user', $author, 'field_user_main_image', $settings));
}