Once you link your Facebook account to your Mafia Rivals account, it will automatically grab your current profile picture from Facebook. You can always reload your Facebook account on your Mafia Rivals account to refresh your in-game photo to match your current Facebook profile picture. 

To choose your profile picture, simply tap on your avatar and click the edit button to the right of your current photo. Next you will see upload where you can access your photo gallery. If you are having trouble uploading a photo, head to "Setting" --> "Privacy & Security" --> "Photos" and select Mafia Rivals to allow access to your photo library.