<?php

$imgFolder = "../images/";

if(file_exists($imgFolder . $_GET['p'])){
    header("location: ".$imgFolder.$_GET['p']);
    exit;
}

$src = 'http://www.mywebsite.com/'.$_GET['p'];
if (!$img = imagecreatefromgif($src."/preview.gif")){
    $img = imagecreatefromgif('http://www.mywebsite.com/na.gif');
}
// some image processing code like resizing,etc
// ......

// save image at $imgFolder.$_GET['p']

header("Content-Type: image/gif");
imagegif($img);
// Remove image
imagedestroy($img);
?>