In the skin gallery, you will see many images. One of them is a screenshot, and one of them is the actual skin file, the screenshot should like like the image below

There is no need to download the screenshot. It is simply to show you what the skin looks like. The image you need for minecraft is this small one.

Right click it, save image as, and BINGO! There ya' have it.

It is very simple. The first thing you need is the skin. Here is an example: When you have it, go to minecraft.net and sign in. Click profile, and upload the skin.