I often ask clients to take progress pictures following the advice below in order to make it easier to visualize a difference in physique.
1- Use similar lighting. (natural light is best) Can even take this one step further to say same time of day to catch similar natural light.
2- Use the same angle to ensure consistent perspective. I encourage having someone else take the pictures or use self-timer. Selfies in a mirror are not a true representation because we all know how to work those angles to make pics more flattering.
3- Wear the same clothes in comparison pictures. This makes comparing the pictures more reliable if we are consistent and not distracted by different outfits which can misconstrue perception.
4- Use a full body shot.
5- Use a forward, side, and back facing pose to see a full, 360 degree view. (or do poses according to your sport, i.e. bikini, figure, etc.)
6- Take pics at similar times of day. Sometimes we are bloated in the evening after a full day of eating, so I encourage morning progress pictures on an empty stomach.
Using progress pictures with clients is a useful way to indicate body composition change. We know the scale does not always tell the full story. I like to track other data too such as girth measurements, qualitative feedback stating improvement in quality of life from a client, quantitative feedback like increase in load/reps lifted in the gym, consistency in positive habits, improvement in form via recording their lifts at the gym, etc.
There are plenty of ways to track and measure changes/progress of a client. Sometimes results are tangible and visible to the eye, but sometimes the most important changes are internal and just as rewarding for the coach and client. This is why a feedback and open communication between a coach and client is crucial.