You need to get the filename from the database, not $_REQUEST
.
function delete(){
@include 'config.php';
$comando = "SELECT Nome FROM images WHERE Id = '$_REQUEST[Id]'";
if ($result = mysql_query($comando) and $row = mysql_fetch_assoc($result)) {
$nome = $row['Nome'];
} else {
die("Query failed: " . mysql_error());
}
$comando = "DELETE from images " .
"where Id = '$_REQUEST[Id]'";
if(!mysql_query($comando))
echo "Modifica fallita <br/>";
chmod($path_img,0777);
$path = $path_img . $nome;
unlink ($path);
mysql_close($cn);
}