razor300,
if (file_exists('../files/photo/' . $user['id'] . '.jpg'))
echo '<a href="../files/photo/' . $user['id'] . '.jpg"><img src="../files/photo/' . $user['id'] . '_small.jpg" alt="' . $user['name'] . '" border="0" /></a>';
else
echo '<img src="Путь_до_картинки" />';