You can also use a bit of AR by detecting the face position and angle and communicating with symbols or wording if the picture is acceptable or not.
You probably have seen the Facebook and Snapchat dog, bear faces and various Halloween hats. At some point the AR effect disappears when your head is in a wrong angle or outside the desired area. So if you use the same type of effect with relevant icons that the photo is or isn't ok to take then this is another solution and no need for any how-to screens explaining something what can be adjusted on the go.
Use a semi-transparent overlay when reflecting the direct camera view with an area in the middle where the head should be at. Please see the attached image.