How to add detail to a minecraft skin?

I've tried gimp, the smallest brush size I can get is the same size as the massive pixels that make up the skin itself. I'm not about to buy photoshop just to edit a minecraft skin.

Now, I've seen and used a skin that is highly detailed; complete with drawn and outlined fingers, round cartoony eyes that were more than just 4 huge pixels, visibly wrinkled clothes, and so on.
I look on video tutorials, and all they tell you is how to add shading and variation of skin tone. Their idea of HD seems to be "not one solid color". I wanna know how to do this:

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