Well, thanks for this chapter then! I think there are other free Photoshop alternatives like SAI, if you don't want to use GIMP. I've never needed to use it, but I've heard good things about it
On another note, please do not use SAI for any image editing as SAI is a painting software. It lacks the functions to edit manga properly. GIMP should be able to do most of the stuff Photoshop does, though it's indeed not the best tool of image editing.