How do you put the individual pieces of a sprite together?

I was following along with a tutorial from YouTube ( https://www.youtube.com/watch?v=eXIuizGzY2A ). He shows you how to use the Sprite Editor with an image, and the individual parts of the sprite are separated. At around the 2 minutes 40 seconds mark, he goes to the Skinning Editor, and the sprite is put back together. How do you do that? I don't have Photoshop, so I'm using a PNG image with the sprites body parts separated.