Product Photo opens as new page, not light box

How do I tell if this is a wordpress problem, plugin problem, or template problem: used to be a user clicks on a product page photo and it opens in a “light box” over the page that users can “X” out of, but now it opens the .jpg file as a page such that users have to hit the back arrow. I’d like to understand this and fix it without contacting the original? developers if possible.

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