the short answer
Client progress pictures work when they are standardized and private. Have the client shoot front, side, and back in the same bright, even light, the same distance, the same fitted clothing, and the same time of day every two to four weeks, then compare each new photo against the baseline at the same scale.
Progress photos are sensitive personal data. This article is general guidance for coaches - default to private storage, get explicit consent, and follow the privacy rules that apply where you and your client live.