Portrait screens are quite normal of course, since pretty much all mobile devices are held vertically. The popup will always scale the image to fit inside the viewport and center it. Likewise, on desktop screens, vertical images will scale from top to bottom, and the image will be centered horizontally.
What you are asking for, is something like the screenshot below. I really can't see how this is useful, it's not intuitive in regards to navigation and gestures, and it's not a pattern the visitor is used to. It can perhaps be set like this with some custom CSS, but I can't recommend it.