Ok, I managed to solve this in the following way.
You can store the e-mail address in the success message through the back-end with [email].
Then you are able to access the e-mail through JSON, with the following command:
var message = data.message;
If you put this in the on_sent_ok function, you get the full success message. I have put a : in the success message, so that I could split the variable on this sign and put it in the e-mail variable.
The code to get the tracking pixel I was looking for is shown below. I hope it helps someone.
on_sent_ok: "var email = data.message.split(':');email = $.trim(email[1]);var now = new Date();now = now. getTime();var div = document.getElementsByClassName('wpcf7-response-output wpcf7-display-none wpcf7-mail-sent-ok')[0].innerHTML = '<img src="https://tl.tradetracker.net/?cid=10978&pid=16524&tid='+now+'&eml='+email+'&descrMerchant=viacontactform&descrAffiliate=viacontactform" alt="" width="1" height="1" border"0" />'"