Choose a photo where the face is sharp, naturally lit, and not covered by sunglasses, hair, or a strong filter. A relaxed expression usually gives the model more useful detail than an exaggerated pose.
A front-facing or slight three-quarter angle works well. Avoid tiny group photos and heavily compressed screenshots when you have a clearer original available.
One photo is enough for the demo. When using two, choose images with similar lighting so the creative result has a more coherent mood.