<img src="URL_TO_PAP/scripts/imp.php?a_aid={$refid}&a_bid={$bannerid}" width="0px" height="0px">
Do not forget to replace "URL_TO_PAP" with the real URL address leading to your installation of Post Affiliate Pro (URL has to contain protocol 'https://' or 'https://').
This impression tracking code could be added automatically by variable {$impression_track}.
In the above mentioned sample impression tracking pixel we presume the affiliate URL parameters regarding "affiliate id/referral id" and "banner id" have not been changed in the merchant panel at Configuration > URL Parameter names.
NOTE: if the particular customer to whom the email is delivered uses an email client that does not support HTML output, then the impression tracking pixel won't be executed.