Assuming you're changing the links on a dynamically generated free hosted gallery, eg. you're changing the links on:
http://www.fhgdomain.com/fhg.php?id=cocksmoker
On fhg.php:
PHP Code:
<?php
// Extract the affiliate id from the url
$affiliate_id = $_GET['id'];
// Insert the affiliate id into the sponsor code
$link_url = "http://www.sponsordomain.com/clickthrough.php?id=$id";
// Print the modified url
print "<a href=\"$link_url\">Click here to check out this fucking amazing sponsor</a>";
?>