Different Image from Carousel to Open in Lightbox

Hello!
I have a carousel on my homepage that features sort of large thumbnails (300×225). When the image is clicked, I have it opening a different image which is the fullsize, watermarked version of the image. The fullsize image opens in a different page. I would like the fullsize images to open in a lightbox. How do I accomplish that? Here is my development area: https://usadancedc.org/wordpress/

Thank you!
-Myra

https://wordpress.org/plugins/shortcodes-ultimate/