display random pics from folder but with 1 pic per visitor

i was trying to make a random display of pictures from a certain folder but i also want to show only 1 picture for each visitor / IP, and i guess this has something to do with 'sessions'. this is the code i tried bu it doesn't seem to redirect me to index2 :

    <?php
session_start();

if (!isset($_SESSION['views'])) { 
    $_SESSION['views'] = 1;
}

$_SESSION['views'] = $_SESSION['views']+1;

if ($_SESSION['views'] == 1) {
    header("location:index2.php");
}
?>

Thanks.