Image heavy page with lightbox

I’m building several pages that will have quite a lot of images on them, most of which are 700kb-1mb each. I’ve got them all in a linked lightbox, and I’ve manually created thumbnails for each that are much smaller (100kb~).

My question is, will the lightbox images all load with the page, or only when the lightbox is opened by clicking an image? Also, is it actually beneficial to use thumbnails rather than the same image, or is that just making the same image load twice basically?