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.
Why would I believe that the ISS is fake? I also believe we landed on the Moon and also that NASA sent two rovers to Mars as well. Space travel actually isn't incompatible with concave Earth theory like some say it is.
As for the reasons why we can see the Moon in the Sky, well probably the same reason we can see the Stars in the sky, it's because firstly we can see about 50% of the stars in the sky at any given point in time from one single angle. This should prove that either the sky is a concentric sphere and rotating inside the Earth or that the Earth is a concentric sphere and rotating itself. It can not be a concentric sphere which is not rotating nor can it be flat unless the sky itself is flat and also rotating in two opposite directions but even then it still doesn't line up with a flat earth. If you believe in any flat Earth map then you also must believe that you physically grow in size the closer you get to the poles, plus I'm pretty sure the idea of an Ice-Wall surrounding the Earth is a pretty rediculas notion seeing as how we have actually mapped the entire Earth and have not seen anything and with the thousands of satellites orbiting the Earth in every direction (East, West, North, South) I'm pretty sure we would have satellites fly off into the universe every day if it was flat. Simply put satellites are an impossibility if you believe in the flat Earth theory.
As for the ISS photo, I suppose you are right in that I do not in-fact know what angle the ship was pointing at. I leveled the camera assuming the horizon should be below the thrusters assuming the thrusters were level with the Earth as they almost always are. However I have done my own test to measure the curvature of the Earth and have personally determined the Earth to curve up. Cyrus Teed of course did the same thing over 100 years ago and found out the same thing.
As for light itself I am a little unsure of how it works, for starters light only works be reflection off surfaces of objects so if the angle is up then the reflection angle will bounce at an upward angle if the Earth is concave so to your perspective any light rays that are higher then your eye level will also be higher then your eyes so you would not be able to see them but that would also explain why you can in-fact see further when you go on higher ground. If Earth was flat/convex then the horizon should go significantly lower then eye level at very low altitudes because light always travels in straight lines away from the surface it bounces off of if the object it hits is concave it will travel higher then a level observers sight where if it is a convex or flat surface it should always be lower then it is. True objects should always therefore be subjectively lower then level if they are flat or convex and right at eye level if it is concave.
As far as images go, yes we cannot exclusively base our opinions based on visual imagery. Of course the only thing I really have to compare Earth to is footage of the Moon and Mars and based on what I have seen the Moon seems to curve downward based on what I have seen of their horizon, like take this image, it definitely looks like the Moon curves downward as the horizon line is well below eye level compared to the Earth where the horizon is always at eye level.
About this bit
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.
I think this is actually impossible because your pipe would shrink the further and further it went into space. In fact space is big, like 6 light days across by my measurements where Earth is literally only about .134 light seconds across on the surface. Of course this is also why you can't in fact see contents above your eye-line because light only travels so far and the reflections of light after travelling 12 days from one end of the Universe to the other would literally only appear like points of light in the heavens or stars after it would have been diminished quite badly. I'm not exactly sure how far light can travel anyways before it diminishes into the invisible background radiation level though. Probably not very long, only the most reflective points on Earth become stars in the heavens, all other energies seem to be absorbed by these point light sources. Of course planets are layers.