Here you have a more in deptht explanation of the lightbox functionality.
I would suggest doing the following
Add a classic image element. on the bottom of the image element add a class: light-1 - turn on the link functionality and add to the href box a link to the image itself.
Add a second image element and do the same but with a different class, example light-2
Do that as many times as you need.
You will than add a raw code element, and add multiple lightbox selectors there, one for each image/class.
So in our example you would add:
Hope it helps