Photobooth is an Android application, connected to an external button and an external camera. The Android devices uses its front facing camera to show a preview image and when the button is pressed, triggers taking a photo with the external camera (and displaying the resulting image).
button -- Raspberry Pi ----> Android tablet <----> GoPro camera
or
button -- Raspberry Pi ----> Android tablet <----> Android camera
or
button -- Raspberry Pi ----> Android tablet with front-facing camera
 |
 |
 |
 |