I too also had this issue and based on the solution provided previously. Create the following typology rule to automatically add the pixel just before the body tag to all email using the rule.
if (delivery.targets.filteringSchema == "nms:recipient") {
var html = delivery.content.html.source.toLowerCase().indexOf("<body>");
var openFormula = getOption('NmsTracking_OpenFormula');
openFormula = openFormula.replace("$(urlId)","1");
var pixel = openFormula;
var newHTML = insert(html, pixel, delivery.content.html.source)
delivery.content.html.source = newHTML
}
function insert(index, string, originString) {
if (index > 0)
return originString.substr(0, index) + string + originString.substr(index, originString.length);
else
return originString;
};
return true
