When you're on your affiliate program, you have to get your link first, all the code is there. A tracking pixel sounds like LinkShare to me, though it could be B&N, Amazon, etc. If it's LinkShare, choose the "Get Links" tab, select your program, find the banner you like, copy the code under it, and paste it somewhere in your index.php file in between
</div> tags. If the program has selected to allow you to use a 1 pixel image with their banner, then the appropriate pixel image code will all ready be with the ad code. If they haven't, then you could certainly add another banner's 1 pixel image to your current ad code, but it won't track.