How to download a skin in Minecraft?

I'm able to get the skin and see the skin and I'll have it for a while. Then my brother will put Ina different skin using the method I told him about. (We use skindex) but the problem is when I want to change back to my fave girl skin that I've already used before, I can't find it! Even if I type it's name on skindex it won't come up. I don't think I've ever put it in a folder but I don't know.