Don't. Just save it as it is and it includes the whole pic, well at least that's how it happened to me.
Mine has stayed as the corner for a while though?
It won’t let you on mobile, but if you do it on desktop you can crop and change the position of your picture so it’s centered.
At first I tried real hard to take the whole pic in, it didn't work, then I thought "it's better than having no pic at all right?" and saved it. It took the whole pic. So dunno man ╮(￣▽￣)╭
Whenever I try to change my profile picture it only captures the top left of the picture and I cant move the box to the middle.. how do I fix that?