So when I wrote the above reply, I was clearly not aware of how to actually use PHP variables. I've taken some time to sort out a couple details and it became clear how easy my next step was.
In this wonderful code from Fredxeric, I modified the second last line:
to be simply:
and in my PHP page in Joomla, I use an url with the pixelpost id renamed 'ppid' and then code in the Joomla content to read 'ppid', sanitize it and spit it back out in an url which forms the link to Fredxeric's code. It's:
<?php $ident = JRequest::getVar('ppid',0); @readfile('http://mysite.com/photoblog/external_thumbs_linking.php?id='.$ident); ?>
I hope that's clear. Thanks again, Fredxeric, for the huge help!