function set_random_representant($categories)
{
  $datas = array();
  foreach ($categories as $category_id)
  {
    $data = array('id' => $category_id,
                  'representative_picture_id' => NULL);
    array_push($datas, $data);
  }

  $fields = array('primary' => array('id'),
                  'update' => array('representative_picture_id'));
  mass_updates(CATEGORIES_TABLE, $fields, $datas);
}

if (($category['nb_images'] > 0) && (isset($category['representative_picture_id'])))

$query = '
SELECT id,name,uppercats,global_rank
  FROM '.CATEGORIES_TABLE.'
  WHERE id IN ('.implode(',', $associateds).')
    AND (representative_picture_id IS NULL
      OR representative_picture_id != '.$_GET['image_id'].')
;';

SELECT id, name, date_last, representative_picture_id

while ($row = mysql_fetch_array($result))
{
  // First search representative picture if defined for current category
  if (isset($row['representative_picture_id'])) {
    $query = '
    SELECT path, tn_ext
    FROM '.IMAGES_TABLE.'
    WHERE id ='.$row['representative_picture_id'].'
    ORDER BY RAND()
    LIMIT 0,1';

    $element_result = pwg_query($query);
  }
  
  // Then if representaive picture is not specified or not found
  // search a random picture among children categories (recursively)
  if ((!isset($row['representative_picture_id'])) || (mysql_num_rows($element_result) == 0))
  {
    $query = '
    SELECT path, tn_ext 
    FROM '.IMAGES_TABLE.' AS I
    INNER JOIN '.IMAGE_CATEGORY_TABLE.' AS IC ON I.id = IC.image_id
    INNER JOIN '.CATEGORIES_TABLE.' AS C ON IC.category_id = C.id 
    WHERE uppercats REGEXP \'(^|,)'.$row['id'].'(,|$)\'';
    // we must not show pictures of a forbidden category
    if ($user['forbidden_categories'] != '')
    {
      $query.= '
      AND C.id NOT IN ('.$user['forbidden_categories'].')';
    }
    $query.= '
    ORDER BY RAND()
    LIMIT 0,1;';
    $element_result = pwg_query($query);
  }