Fullscreen Image

What is the easiest way to make an image fullscreen? I don’t want it to be a background image. I’d like to be able to click on a photo to make it fullscreen and then click again to go back to the web page. It’s just one image on one page. Is there a plugin or some code I can use. As easy as possible — or I’m open to another solution. https://energytothepoint.com/mystic-healing-room/
Thanks, Barbara