How to get the browser to cache images, with PHP?

I'm totally new to how to cache images.

I output all images in a gallery with PHP, and want the images already shown, to be cached by the browser, so the PHP script don't have to output the same image again. All I want is the images to show up faster.

When calling an image I do like this:

<img src="showImage.php?id=601">

and the showImage.php-file does:

$id = (int) $_GET['id'];
$resultat = mysql_query("
    SELECT filename, id
    FROM Media 
    WHERE id = $id
");
$data = mysql_fetch_assoc($resultat);

...

//Only if the user are logged in
if(isset($_SESSION['user'])){
    header("Content-Type: image/jpeg");

    //$data['filename'] can be = dsSGKLMsgKkD3325J.jpg
    echo(file_get_contents("images/".$data['filename']."")); 
}