To customize landing pages for WHMCS, you change the PHP code in the unencrypted aff.php page, located in the root of your WHMCS installation.

Code lifted from this thread:
http://forum.whmcs.com/showpost.php?p=112999&postcount=6

<?php

define("CLIENTAREA",true);

include("dbconnect.php");
include("includes/functions.php");

if (isset($aff)) {
    update_query("tblaffiliates",array("visitors"=>"+1"),array("id"=>$aff));
    setcookie("WHMCSAffiliateID", $aff, time()+90*24*60*60);
}

//Custom page code
$destination = "";
if(isset($_GET["p"]))
    $destination = $_GET["p"];

header("HTTP/1.1 301 Moved Permanently");
header("Location: ".$CONFIG["Domain"]."/".$destination);

?>

Links go like this:

<a href="[AffiliateLinkCode]&p=samplefolder/samplepage.php">Sample Page Direct Affiliate Link</a>

This allows you to set specific banners, links, etc. for specific landing pages.