Head over to your Account Settings, and click 'UPLOAD PICTURE'. It will save automatically once you've successfully uploaded and cropped the image, and then clicked 'SAVE" in the pop-up window.
If you make any more changes to your profile, be sure to scroll down and click 'SAVE PROFILE' in order to make sure your updates have been saved.
Navigate to your profile by tapping the little person icon in the bottom right of the app, then tap the cog to go to your settings.
Tap on the default picture to add your own image from your camera roll.