On Instagram you can see profiles with vertical photos that cover a larger area of the phone. Some people to show the picture taken in full vertically decide to save it with a white outline, but this is not the best way to save photos on Instagram.
Simply click on the icon you see selected in red to make it vertical. As a preview you will see it crushed in a square but when you go to publish it will be displayed in full. Of course again there is a height limit of the photo that you can not change but you can always reduce it by going to make "pinch" on the screen.