grazie,

risolto cosi':

<?php
include "config.php";
$query = mysql_query("SELECT id_user, date FROM messaggi ORDER BY date DESC limit 1");
while($riga = mysql_fetch_assoc($query))
{
$id_user = $riga["id_user"];
}
if(!isset($_SESSION["id_cartella"]))
$_SESSION["id_cartella"] = substr(date('i'), -1, 1);


$cartella = "fotos/".$_SESSION["id_cartella"] ;
$image_src ="fotos/" . $_SESSION["id_cartella"] . "fotos/"."1" . ".jpg";
echo "<img src=\"$image_src\">";

?>