How to make thumbnail images pop up to full size

I want to have thumbnail images on my homepage that will open as popups when clicked to the full size images. I am using the divi theme from elegant themes.

I put up pictures of the thumbnails but they popup to larger versions of that, not of the full size images. I also tried making a portfolio but that opens up each image in its own page, not as a popup.

Any help would be appreciated!