Seriously, I've seen some amazing stuff. I talking about the pictures of real characters people have done (Mario, Megaman, Yoshi, etc.). Do you find an image and open it up in MS Paint or something and do it pixel by pixel or what?
But seriously, when I played Classic (and made a ton of pixel art), I would just open up a reference image and match the pixels the best I could. I think there are programs that actually turn images into Minecraft block representations, and programs that go one step further by building those images in-game for you!