If you're talking a professional photo shoot, those can go on for quite some time and it may be hard for the model to maintain his erection. Some guys use cock rings to help. Others just find the ring hot in and of itself and like the look. But I don't think seeing a cock ring in a pic means that he's going to have a hard time having, um, a hard time.
That said, a cock ring has the opposite of the desired effect with me.
Kevin Slater