You'll have to check your router manual how to do that since it varies from router to router - it's usually something along the lines of accessing the router's control panel via a web browser using it's internal LAN IP and whatever port it's running the control panel on, then adding the option in there.

Since it seems you haven't done that before though, it appears that the default is to forward port 80 to your machine, hence why we're seeing the images already The problem is that it's not forwarding internal port 80 requests to the external IP back to your PC, only external requests to your external IP.