I’m not sure if it could be done with the current setup. The only way I know you could add custom content within the prettyPhoto pop up image is by manually adding the html content/photos. They should have some documentation on this – http://www.no-margin-for-errors.com/projects/prettyphoto-jquery-lightbox-clone/
This portfolio just shows the featured image when clicked, but it doesn’t really go beyond that. I would recommend searching WordPress.org for a more complex portfolio plugin that can offer more functionality, as this one is pretty basic.