There is this guy I know, he's called Lassen, from what I've seen, he's very good, feel free to have a browse through his skins and ask him to make one. His name is lassen and you can find his account by google searching: "Who made the half-craft skins Minecraft"
Also, check out my blog where I include a link to his skins which you can customise.
GIMP is a good editor, Paint.NET is reasonable, but you should search Minecraft skin maker and find easily customisable and inter-changeable ones to better help create your skins.