How To Paint My Own Sky-textures?

HiI searched for hours but can't find any topic about this.Is there any description out there on how to paint Skytextures for FS9?What are the Pixels at the top of the texture for?and are the ones on the bottom for the reflections on the water?I would like to try to paint my own set of sky-textures, so i am very interrested in finding out how to do it right.i hope somebody here can help me.regards M.