You can also use inline php:
Code:
<?php
$country = wp_ozh_getCountryName(0) ;
switch ($country) {
case "Canada" :
?>
<a target="_blank" href="http://www.affiliate.com/fs-bin/click?id=referrer_ID"><IMG alt="Canadian Banner" border="0" src="http://images.affiliate.com/outsidebanners/Homepage-120X600.gif"></a><IMG border="0" width="1" height="1" src="http://ad.affiliate.com/fs-bin/show?id=referrer_ID">;
<?php
break;
default :
?>
<a target="_blank" href="http://www.affiliate.com/fs-bin/click?id=referrer_ID"><IMG alt="American Banner" border="0" src="http://images.affiliate.com/outsidebanners/Homepage-120X600.gif"></a><IMG border="0" width="1" height="1" src="http://ad.affiliate.com/fs-bin/show?id=referrer_ID">;
<?php
}
?>