Tracking scripts don't need to do anything but read the hit in the access log. That said, it's also trivially easy to output the bytes for a 1x1 invisible gif as the response from your tracking endpoint.
It is easy, but is it fast? Serving a static GIF has an overhead of ~0, and it doesn't need any executable beyond the HTTP server to run on the machine, except to move the logs out for processing (syslog, rsync, what-have-you, in most cases out-of-band wrt the HTTP server).