Some Advice on affiliate

Hi i am looking for some advice on adding an affiliate networks code to my cart. the below is what i need to add i am just a little unsure which file i should edit i am also using a custom theme.

Can anyone give me some advice.

<img src='https://www.tagserve.com/saleServlet?MI ... RID=<input orderId!>&ORDERAMNT=<input order amount!>&NUMOFITEMS=1&CUR=<input currency code!>' border='0' width='1' height='1'>

Thanks.