Hmm..i tried everything to get url to user image.The perfect solution was fql use like this->
$fql_b = 'SELECT pic from user where uid = ' . $user_id;
$ret_obj_b = $facebook->api(array(
'method' => 'fql.query',
'query' => $fql_b,
));
$dp_url =$ret_obj_b[0]['pic'];
replace pic by big,pic_square to get other desired results. Hope IT HELPED....