Xanthippe,
Oh I love it when someone plays Devils advocate because at times I do it too.
And that is a very valid question you ask. So lets break it down. If someone claims they have a real picture of a ghost then obviously they would not object to others trying to prove their claim. So first thing is they would be willing to let others exhaust every possible avenue available to them to prove it was a fake. That would be step one to me in giving any credibility to such photo.
If after intense scutiny by professionals in many fields, such as Photography, Special effects (specializing in mirrors and lighting etc.) could find no evidence of tampering with such image then my next question would be to ask to see the area or room where the picture was taken.
If there was indeed a "ghost" or something in that area then it should be able to reproduce itself or left residual of some form from its earlier encounter. The principle of displacement would not allow "nothing" to take the place of where something used to be.
In other words if you fill your bathtub to its limit with water, then put a small toy boat in it, it will only displace the amount of water equal to the volume of the boat. No more no less.
So if something metaphysical was there it had to displace some surrounding air or matter of some form to take its place even if for only a brief time.
Even string theory would indicate that something changed in order to accomdate this.
So logically next step would be to get readings of the surrounding air, earth, radioactivity, radiant heat tests and imaging etc. and look for any "metaphysical/paranormal fingerprints" that may have been left behind.
Physical creatures leave something behind everywhere they go even if not intended, such as footprints,fingerprints,blood,mucus, urine etc. so it would only be natural to assume that whatever this "Ghost" was that it left something behind.
And while the tests were being performed also interview the individual(s) and ask every perceivable question that could help shed light onto this claim such as how long did the ghostly form stay? Did it try to communicate either directly or indirectly with you? Did it just disappear or did it head off into a particular direction? Have you ever encountered anything like this before? (Many other questions come to mind but won't list them in interest of time but you get the general idea)
And if after exhausting every scientific test/method available (or currently known to man) and the results were the same about something being there, then I would conclude that a paranormal/metaphysical being was in existence.