grazie, ho risolto cosi':
<?php
$connessione=mysql_connect("localhost", "
$selezione_db=mysql_select_db("guest_book", $connessione);
$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'];
}
$id_cartella = substr($id_user, -1, 1);
$cartella = "fotos/".$id_cartella ;

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