What is a good privacy-first onboarding flow for a face-analysis web tool?

I am exploring a small browser-based AI tool that analyzes a selfie to estimate face shape and then suggests hairstyles and eyewear. The project is available at https://aifaceshapedetector.net/. For a mobile-first product that keeps the image local, what onboarding and documentation patterns would make the privacy model clear to users while keeping the first result fast?