problem with MergeBlock (limiting number of items)

i tried to do the following to display only 15 thumbnails:

$sql = "SELECT * FROM ping_p2w WHERE wid=".$wid." AND relationship='friend' ORDER BY rand(".time()." * ".time().")";

$tbs->MergeBlock('members',$db->conn,$sql,1,15);

only one photograph gets displayed - i have used this SELECT elsewhere so i believe the problem is in my MergeBlock call ??

For page #1:
  $tbs->MergeBlock('members',$db->conn,$sql,15,1);