勿体ないので掲載。

$id = (int) $_GET['id'];
define('DB_HOST', 'localhost');
define('DB_USER', 'username');
define('DB_PASS', 'password');
define('DB_NAME', 'dbname');

$column = 'data';
$table = 'table';

$dbh = mysql_connect(DB_HOST, DB_USER, DB_PASS);
mysql_select_db(DB_NAME, $dbh);
$result = mysql_query("SELECT `{$column}` FROM `{$table}` WHERE `id` = '{$id}'", $dbh);
$row = mysql_fetch_array($result);

header("Content-type: image/jpeg");
print($row[0]);

以上。