How do I edit textures?

[gimpz]

I took a look at the editor but it feels like I have no clue what I'm doing in it so would someone kindly explain how to edit Amelie's model in the game world? I can get any software necessary to do it and have worked with texture editing for the game Oblivion, though I suspect it's a bit different for KB. I want to make her horse black with dark green armor or whatever looks good, then I will release it as a mod.