Not a bad idea, at least for the beginning. I never heard IFTTT before. Just checked it. May work. So the idea could be:

• There is a bot that randomly texts a message in a Signal Group that one or all users shall send a picture now.
• The group may have autodeletion of the pictures (after 24 hours).

Limitation:

• No "Double Picture (at same time)"
• There is no measurement by software is a picture is too late
• There is propably no direct notification only to specific users.
• There is no gaming thing where you get credits, when you are on time.