There are two issues to consider in this case. The first is whether he had the right to make the images and the other is whether he had the right to leave them in the prayer room.
I agree that he should have the right to express his opinions any way he chooses and proclaim them openly but it seems to me that the airport has the right to decide what they want in their prayer room. It seems to me that he is infringing on the rights of the managers of the airport by using their property to express ideas they didn't agree with.
However, from reading the story I get the impression that he is being punished for making the images, not just for leaving them where he did. If that is the case then I agree that this is suppression of free speech.