There isn't any circumstance in which a photograph should cost $6.5 million. It lacks any justification. Even if the photograph held substantial evidence in regards to a criminal matter or life altering discovery, the photograph's worth could never amount to such an insane sum of money. Unless the picture is worth that amount of money to some one personally, it is unjustified, seeing as printing a picture costs considerably less than a single dollar.
On one hand, there is an argument to made that, if someone has paid a given price for an object, then that object is worth that much money, because indeed the market has already borne it out. Yet it is impossible not to scoff at the ridiculous price of this photo. Granted, it is a really beautiful and interesting photo, but in the world of digital enhancement that we live in, such visual effects are not worth anything close to that price tag.
Whilst $6.5 million is a lot of money it's value cannot be compared to something that is irreplaceable. A photograph that captures a very important moment in time that cannot ever be captured again can be seen as almost priceless. So paying a lot of money for such a thing is not to be ruled out.
If a person has $6.5 million dollars and chooses to spend that money on a photograph, that is that person's prerogative. Photographs are artistic and historic representations of the world just as Picasso's paintings and Michaelangelo's sculptures captured history in an artistic manner in their time. People would never question paying that much money for one of those artist's pieces.