All that’s required are two or three digital shots showing a couple of facial angles ie. straight on and a 45 degree angle. It’s important that the shots show the expression that’s required in the illustration ie. if the expression is a smile, the photo should be a smile, if grouchy then the expression needs to be grouchy. If it’s a full body caricature, it’s not necessary to have a full body photo – just a good description of the body shape, height and the sort of pose that would suit.