I think he means that he wants to know how you can tell that the pictures are fake.
I mean, really, you look at that picture of Hoth. If I didn't already know that Hoth was fictional from Star Wars, and I saw that picture, I'd be pretty hard pressed to believe that it is real. Because in reality, with out knowing the truth behind it, you can't be totally sure of anything. The only option you have is to accept it until you can prove it's fake.
So then the question becomes how specifically is it fake without knowing beforehand that it's Hoth from Star Wars?
In the case of midgard's pictures, those pictures are indeed real, but not real in the sense that they for sure prove the existence of the ice wall. The last picture he used, has specific tell-tale signs that we can use to tell that it was modified.