Adding images to porfolios

Hi. I used the code on the help file to generate some portfolios and set my desired feature image, but I cannot figure out how to add photos to the portfolio. I’ve tried switching from standard to gallery, but I don’t see an option anywhere to add more photos. When I click the “add media” button to ad photos, they don’t display like I want them to.

The end result I want is when someone clicks on the featured image to enlarge it, I want them to be able to click the arrow to the right or left to see more images in the portfolio.

Thanks!