Does every picture have a unique what3words address? Or is it based on
the location of the picture? What if the initial location is wrong and
gets corrected? Does the picture’s what3words ‘address’ change as well?
No, it is not unique. Several photos may have the same W3W address if they are taken from a similar location.
Yes.
Yes, the W3W address should change, as soon as the new location is inside another W3W square.