On an IPhone 5, how can I put a picture in a folder?

On an IPhone 5, how can I put a picture in a folder?

What do you mean? Are you wanting to add picture to a folder on your Springboard/Home screen? If so, you can't. Apple doesn't want anyone to customize their phones apart from the background wallpaper.

If your iPhone was jailbroken, you might be able to download a tweak that allows you to do that, but I don't know of any.