Best answer by Mathis
If your old profile image is still showing on your mobile app, chances are that the old image is still cached within the app. Try swiping from top to bottom on your profile page on the phone to see if it updates. If not, restarting the app should do the trick.
If none of this helps, though, try deleting the app, restart your phone, then download the app again from the App Store / Google Play store.