Some advertising networks use tracking pixels <img> tags that send a ping back to theirs servers. In order to connect these pixels to the banner you can use the following Javascript snippet.
<script type="text/plain" data-cookiefirst-category="advertising">
var img = document.createElement("img");
img.src = "url here";
img.style = 'width: 0; height: 0; border: 0';
document.body.appendChild(img);
</script>