Hi,
looks like it's the same template for several page types among category, recent_pics, ... etc.
Give a try to /themes/default/template/thumbnails.tpl

FOUND IT! in /include/section_init.inc.php

// +-----------------------------------------------------------------------+
// |                       recent pictures section                         |
// +-----------------------------------------------------------------------+
  else if ($page['section'] == 'recent_pics')
  {
    if ( !isset($page['super_order_by']) )
    {
      $conf['order_by'] = str_replace(
          'ORDER BY ',
          'ORDER BY date_metadata_update DESC,', ===CHANGE
          $conf['order_by']
          );
    }

    $query = '
SELECT DISTINCT(id)
  FROM '.IMAGES_TABLE.'
    INNER JOIN '.IMAGE_CATEGORY_TABLE.' AS ic ON id = ic.image_id
  WHERE '
  .get_recent_photos_sql('date_metadata_update').' ===CHANGE
  '.$forbidden
  .$conf['order_by'].'
;';

    $page = array_merge(
      $page,
      array(
        'title' => '<a href="'.duplicate_index_url(array('start'=>0)).'">'
                    .l10n('Recent photos').'</a>',
        'items' => query2array($query,null, 'id'),
        )
      );
  }