Sculelos, since when do you believe the ISS is real? That image from space looks totally like CGI. Even if it were authentic, you can't possibly know where the true horizon should be, because you don't know how the camera is pointed or the ISS itself for that matter, as it has been already suggested by others here.
Furthermore, when you said that if 'if it is up it bends down, and if it is down it bends up' you basically said that what we see in images is not real, and we cannot base any conclusions on them even if they are genuine and not fake.
Also, about the concave Earth theory, there is no meaningful explanation why we can see the Moon above the horizon, but we cannot see land which is 1000 km away in the sky above the horizon. What allows us to see the Moon, but prevents us from seeing continental land masses in the sky?
Not to mention that if Earth were concave you would be able to extend a pipe up into the sky and reach another city eventually.