It is already possible to know which devices are near to other devices. So if somebody steals your phone then that device will enter into connections not previously established and hence trigger some other protocol that the original user needs to perform. As you say, perhaps a new photo, although what about taking a photo of one of the selfies in the stolen phone?
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The real owner could take a picture of himself showing in a paper the most recent hash in the blockchain. But yeah, this system becomes more and more complex. So it is difficult to be 100% and more difficult considering the advances in artificial intelligence that will come in the future.