using borderpx theme, want to create a link in the portfolio (gallery)

I’m using the borderpx theme on my client’s site here: https://lizmurraydesign.com/portfolio/
She wants to extend the functionality so that when you click on the main image, it goes to a specific landing page for that shot so she can add more information. The pages already exist, because each image is a post. How would I know/create/link that main image to its corresponding post/landing page? (for instance, this one: https://lizmurraydesign.com/outdoor-spaces/joh-ext-027/ )