Need help with timthumb.php

I have a php file that works like timthumb.php, which takes images from another site and creates a thumbnail with it. The problem is, if the image I'm trying to retrieve doesn't exist, I get a broken image. This file is used in a software I have on a site. My question is, does someone have a solution to the code below, that would allow me to show a default image if one isn't available? I would be forever indebted for you if you can help!

<?php
/************************************************************
  Download Program ThumbNail Function
************************************************************/
   $extension = end(explode(".", $_GET['filename'])); 
 
   if($Size) {$filename=$filename."&Size=".$Size;}
   // Content type
   
    if ($extension == "gif") {
    header("Content-type: image/gif");
	} elseif ($extension == "png") {
    header("Content-type: image/png");
	} elseif ($extension == "jpg" || $extension == "jpeg") {
    header("Content-type: image/jpeg");
	}
   
   
   // Get new dimensions
   list($width, $height) = getimagesize($filename);
   $ratio=$w/$width;
   $new_height = $height * $ratio;
   
   // Resample
   $image_p = imagecreatetruecolor($w, $new_height);

    if ($extension == "gif") {
    $image = imagecreatefromgif($filename);
	} elseif ($extension == "png") {
    $image = imagecreatefrompng($filename);
	} elseif ($extension == "jpg" || $extension == "jpeg") {
    $image = imagecreatefromjpeg($filename);
	}
   
   imagecopyresampled($image_p, $image, 0, 0, 0, 0, $w, $new_height, $width, $height);
   
   // Output
   imagejpeg($image_p, null, 100);
?>

PHP:

 

Use something like this.

if (file_exists ($filename) == FALSE){
	$filename = "default.gif";
}

PHP: