I've got a scanned document image on my test Joomla 3.6.2 site. Here it's URL:
As you can see, on the right side there I've got a lightbox module (it's actually a Custom HTML module) that has this code in it:
<p><a class="spotlight" href="images/document2.jpg" data-lightbox="on" data-demo="group:mygroup1" data-spotlight="on"> <img src="images/document2.jpg" alt="demo" width="98" border="0" /></a></p>
My screen resolution is 1366x768. Full image size is W 583px H 800px. When I click on its icon, it opens up, but it's still like 50% smaller, than it's full original size. I made a printscreen image, that I will be attaching here. From what I was told, people who has Full HD monitors (1920x1080) they see the image in its fullness. So it's probably responsive, depending on the monitor's size. Anyhow, I wish to cancel the responsiveness and re-sizing at all. So on every screen resolution (I'm not talking about mobiles here) one would get a full size image after clicking on its icon.