Well it’s quite easy, I just did it in 30 seconds:
Just insert this code into your Ghost admin Code Injection section:
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/fluidbox/1.4.2/css/fluidbox.css" />
<script type="text/javascript" src='https://cdnjs.cloudflare.com/ajax/libs/fluidbox/1.4.2/jquery.fluidbox.min.js'></script>
<script>
$('a').fluidbox({
viewportFill: 0.8,
});
</script>
And you get a nice lightbox for images from within your stories. I used the info from the first link I provided above.