[kh99]

I think you could do something like this:

$orderby = "rand(NOW())";
$cutoff = TIMENOW - (30 * 86400);  // 86400 = seconds in one day

$lpictures = $db->query_read("
  SELECT albumpicture.*, album.*, user.username, user.usergroupid,
  IF(displaygroupid=0, user.usergroupid, displaygroupid) AS displaygroupid 
  FROM " . TABLE_PREFIX . "albumpicture AS albumpicture
  LEFT JOIN " . TABLE_PREFIX . "album AS album ON(album.albumid = albumpicture.albumid)
  LEFT JOIN " . TABLE_PREFIX . "user AS user ON(user.userid = album.userid)
  WHERE album.state = 'public' AND user.lastvisit > $cutoff  
  ORDER BY $orderby DESC LIMIT 0, $tot  
  ");