creating a portfolio page

i’m using a theme with a ‘menu bar’ (if that’s the name) that allows me to have pages like portfolio, about i.e. now, i want to create portfolio page, like photo gallery, with image thumbnails and use lightbox js to open images.

i’ve already created portfolio page that links from menu bar, but how do i go with creating the thumbnail gallery to that page?

i did some research and tryed lazy gallery but didn’t get it to work.

site i’m trying to get this work is https://www.iskoblog.com