For the same token, blockchain will prove a person own something as of that time, if he can store the digital picture in the blockchain. This is doable. As matter of fact, some altcoin is making store extra information in blockchain super easy.
But that's exactly the thing !
The trick with envelope and post does work, because post stamps are legally recognized and respected.
Imagine going to a court and showing what? a webpage of that [Suspicious link removed]pany? an application displaying something?
Even if that is absolutely true - how do you imagine the judges respecting the fact whether you show them on the screen that date your work was "stamped"?
So what?
For them it's just a webpage, or app, whatever, one of millions...
Do you get what I mean?