BaikerZ,
$res=mysql_query("SELECT * FROM users WHERE best='1' AND sex='zh' ORDER BY RAND() LIMIT 1");
$a=mysql_fetch_array($res);
if (file_exists($_SERVER['DOCUMENT_ROOT'].'/files/users/photo/' . $a['id'] . '_small.jpg')) {
echo '<a href="../files/users/photo/' . $a['id'] . '.jpg"><img src="../files/users/photo/' . $a['id'] . '_small.jpg" alt="' . $a['login'] . '" /></a>';
} else {
echo 'Нет фото';
}