how do I reduce my skin's size ?

Description from original post (created 2012-06-03 19:49):

So I made my new skin based on minecraftwiki's template picture, which is 640*320. As you all know a skin's size is 64*32. I thought drawing the skin then reducing it when it's done would be alright but... I used several methods, none work. My skin isn't the same at all. It's supposed to be gray/green and it becomes purple. And of course it's ugly.

So... How do I do that ?