On my ipad 3 with the latest FB app, there is no icon visible to make a picture your profile picture. In the upper right, while a photo is displayed, there is a "done" button only (after you single tap the photo). To make a picture your profile picture, you have to tap and hold on the picture. Then a menu comes up with "make profile picture, save photo, delete photo". Completely not intuitive in my opinion. I was lucky I even stumbled upon it, but that wasn't until I went to the desktop to do it anyway.