the short answer
To track client progress photos, have the client shoot front, side and back in the same light, the same spot, the same time of day and the same fitted clothing every 2-4 weeks, keep each set on a private timeline next to weight and measurements, and compare new photos side by side with the baseline at the same scale.
Progress photos are sensitive personal data: get the client's written consent before the first photo, keep them private by default, never reuse one for marketing without a separate opt-in, and follow the privacy rules where you and your client live (more in client data privacy for online coaches).
We build Coachway. In it, clients add photos through image fields in your check-in form, the photos sit on the client's private timeline next to their measurements, and on the client's profile you can compare two dates side by side.