adding more Images to an expansion?

i have recently started creating my own portfolio website
and i used the free template called :Animation Studio Website
but a few of my projects have multiple shots / images, how can i add more images to the window that appears when you select an image thats in my gallerys section?

please let me know if you need more info or a clearer explanation

my site is: https://jeffreyfrias.wixsite.com/portfolio

Hi,
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