You can change your portrait image on your Passport screen. The portrait currently being used will always appear highlighted. After choosing the portrait you would like to use, you can confirm your selection.
If you are connected to Facebook, you can choose to show your Facebook image to your Facebook friends by toggling this ON in the selection screen.
This is selected by default, however, you can disable it at any time in the future.
Whatever you decide, you are required to choose an avatar photo as this is the image that will be shown to players that are not connected with you on Facebook.