Pop-up Gallery of Featured Images from main posts

Hi,

I have a site that displays photographs on the main page with an infinite scroll. The photos that are displayed are the Featured Image of each post (with the content text underneath).

I’m trying to get it where when you click on an image an image gallery pops up of all the featured images on the home page so far. (Pretty much what happens with the http://www.humansofnewyork.com site, but I can’t figure out whats going there code-wise) I’ve tried a few plugins but I can’t figure out how to dynamically create the images in the gallery from what is posted on the home page.

Basically I want a gallery that includes only featured images from posts in a certain category. All my searches have turned up very little. Is that possible to accomplish?

Any ideas would be greatly appreciated. Thank you!!