http://phpsnips.com/473/Cache-PHP-Generated-Images#.VBG--vmSySo Stops PHP from sending a fresh version of an image if the browser already has it in its cache.

cache_generated_images.php

<?php 
header("Cache-Control: private, max-age=10800, pre-check=10800"); 
header("Pragma: private"); 
// Set to expire in 2 days 
header("Expires: " . date(DATE_RFC822,strtotime(" 2 day"))); 
if(isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])){ 
  // if the browser has a cached version of this image, send 304 
  header('Last-Modified: '.$_SERVER['HTTP_IF_MODIFIED_SINCE'],true,304); 
  exit; 
} 


// Generate an image below either using GD or a file reader such as: 
// readfile(), fread(), file_get_contents(), etc. 

$path = "/path/to/my/image.jpg"; 
$info = pathinfo($path); 
switch($info["extension"]){ 
    case "jpg": 
        $mime = "image/jpeg"; 
        break; 
    case "gif": 
        $mime = "image/gif"; 
        break; 
    case "png": 
        $mime = "image/png"; 
        break; 
} 
header("content-type: $mime"); 
readfile($path); 
?>