Get a picture in-game and when you upload the skin, put the picture in the zip file. Alternatively, sign in, go to your skin and click "Edit this file" and you should be able to add a picture from there. If you don't know how to take screenshots, press the Print Screen key while in game, then open Paint and click paste, then save
Thanks, I have the picture now but how do I display it? I mean, when you go to the Black and White 1 Sheep Skins, how do I make it so that the skin I uploaded displays there?