How is the popup for tickbox generated?

I have to add some custom buttons to the image(i use tickbox as effect) to look like some design and just can’t figure it out how to add some html to that popup image. How the html from the overlay image is created? I search with “TB_prev”(the class from prev button) in the ngg plugin directory and can’t fins anything.

https://www.ads-software.com/extend/plugins/nextgen-gallery/