Run the challenge for your own group. Anyone can create a private gallery: you get a secret invite link to share and a manage link for yourself. Only people with the invite link can upload or look, search engines are told not to index it, and photo addresses are random. Uploads close at the deadline.
Share the invite link with participants. Keep the manage link to yourself: it lets you change the deadline, reveal, remove photos and archive. This browser remembers it; on another device, open the manage link.
Photos stay hidden until everyone has uploaded. Then we open the gallery together.