This plugin does work and for that I give the author credit. However, there are no instructions. You are advised to view the site the author got the idea from. If you look hard, you can find some options to add to the PHP code. They may or may not work, depending on your theme.
Even though I had large images, the best this plugin would do is magnify the medium sized image from the Media Gallery. And, they were blurry. So, I changed the default image size on the Media Gallery and that broke the plugin. Also, if you refresh the main page, it brings up the gallery view of 1 image, as if you'd clicked on it.