"Large" S3 Images Not Able to Load in PHP - php

Question

I'm trying to use WordPress on the AppFog PaaS system. Unfortunately, AppFog doesn't have persistent storage so all content outside of the database needs to be stored on some external system (like S3). I'm successfully using a plugin which pushes all my WordPress media out to S3, but am having problems loading some of the images.
To investigate, I deployed the following script:
// get the image name from the query string
// and make sure it's not trying to probe your file system
if (isset($_GET['pic'])) {
$pic = $_GET['pic'];
// get the filename extension
$ext = substr($pic, -3);
// set the MIME type
switch ($ext) {
case 'jpg':
$mime = 'image/jpeg';
break;
case 'gif':
$mime = 'image/gif';
break;
case 'png':
$mime = 'image/png';
break;
default:
$mime = false;
}
// if a valid MIME type exists, display the image
// by sending appropriate headers and streaming the file
if ($mime) {
header('Content-type: '.$mime);
header('Content-length: '.filesize($pic));
$file = fopen($pic, 'rb');
if ($file) {
fpassthru($file);
exit;
}
}
}
?>
Which allows me to directly test my ability to read and write an image in PHP. This proxy script works perfectly for images under around 10KB -- i.e. when I open the script in a browser pointing it at some "small" image file in my S3 bucket, I'm able to see it.
However, when I attempt to load a "large" file (anything over 10KB), I get an error. In Firefox, that's:
The image “http://myssite.com/iproxy.php?pic=http://aws.amazon.com%2Fmybucket%2Fwp-content%2Fuploads%2F2013%2F01%2Fmylargeimage.png” cannot be displayed because it contains errors.
I've been wrestling with this for hours and can't seem to figure anything out. I've tried changing the output_buffering to a larger value but that hasn't helped.
Any tips would be appreciated!