Your in-game profile picture is automatically synced with your Facebook account. This means that whatever changes you make to your Facebook profile picture will also take effect on the picture that appears in your game.
It may take a couple of moments for the game to sync with your Facebook account, so you may not automatically see the changes you have made.
Unfortunately, there is currently no option to choose a profile picture if you are not connected to Facebook, but we will be working on this feature in the future.