Change Behaviour of PrettyPhoto to custom url on click

We are using fast-gallery to show images, however we would like to replace the click on photo behaviour of the prettyphoto lightbox. Currently clicking on the image provides navigation. Clicking on the left of the image goes left, right of the image right. We would like to replace this behaviour with the custom_url link from FastGallery. I also need a short explanation on what files were changed and where. Thanks