The Gimp has now a very good “crop” tool.
Hmm, I somehow get the weird feeling this thread is a Turing test. I explain something, you pick the wrong thing to ask back!
=> You’re a machine. 
The correct question would have been “Which paint program did you use?”
Paint Shop Pro 7, but that doesn’t matter. You can see that your input image is wrong even by overlapping it with another window aligned to the edges of some of the cubemap faces and see that they are misaligned.
This gets less and less “OpenGL coding advanced”, please stop it.
ok
I tried to convert cubemap to fisheye by using 6 seperate pictures that form the original fisheye. The probrlem remains
You do something else you didn’t say or you do something wrong. There are not enough informations to tell.
This simply should work if the cubemap data is matching at the borders, your upload routine is right, and you set the texture parameter wrap modes to clamp_to_edge.
If you’re still working on that broken input image, the light white lines were in the image before, probably due to incorrect sampling when sizing it down to thie 800x600 image.
Again, DEBUG!!!
Do you see any artefacts when using 6 times 1x1 images of pure white, pure black, or the debug color I proposed before?
Maybe you start with a working program from the samples I pointed you to and exchange the texture data.
It’s not like this hasn’t been done before.