ok, i discovered this while exploring textures to see if i could make my own texture pack...

go to

 "My Computer"
"C/local disk"
"Users"
"[your username]"
Now at this point we need to show hidden folders: you will notice at the top left of the "My computer" window there is an "organize" button. click that and select "folder and search options". click the "view" tab and in the middle of the list down the bottom, there should be two button options which(if you haven't already done this). click the one that says "show hidden files and folders". now click "OK" down the bottom of the window.
"AppData"
"Roaming"
".Minecraft"
"bin"
Now right click on the "minecraft" jar file and open with zip genius or some other program like it that opens .zip files and things like that. now go down till you reach the group of .png files near the bottom of the list. when you find the one you want (e.g. zombie.png) extract it to desktop(that way you wont lose it). then make it your skin on minecraft! it's that simple! alter them as you like!

hope you are satisfied with this!

-TheMedic1231, your friendly neighbourhood spider-man!

Files: 

pigzombie.png