The trusted source of time is the Stellar blockchain, and the last Bitcoin block hash, giving us a ~10 minute period wherein the photo must have been taken. Essentially, we abstract the time synchronization issue away from the photo taker to an external trusted source.

The Bitcoin block hash cannot be known before it's calculated, and a new one is calculated every 10 minutes. Works great.

It also reminds of this paper I read which uses blockchains to capture the video frames from car dash cam for insurance purposes. I wondered about the same clock synchronisation issue https://ieeexplore.ieee.org/abstract/document/8752067

Presumably each car handover is conducted with an internet-connected cellphone, making it trivial to get a trusted timestamp accurate to a second or two.

It's complex to synchronize clocks perfectly, but to within a few seconds is easy and there aren't many cars that could rack up an appreciable mileage in such a short time.

You need some trusted source of time to make this work. So it won’t be completely trust less system.