The downloaded skins arrive at skins_x (low res) and skins_y (high res) but the automatically downloaded skins are not intended for your own use. You should normally just let LFS handle those folders - really they are just data folders to cache the skins you encounter online.
The skins you can select are usually in the skins_dds folder. That is, unless you select to use uncompressed skins, in which case you can see the skins in the skins folder.
If you download a skin jpg from a website (sych as this forum) just put it in the skins folder, it is automatically converted to dds and you will be able to select it in LFS. If you move an automatically downloaded skin from the skins_x or skins_y folder, it is a dds file so you must put it in your skins_dds folder to select it in game. But be careful, they are often other people's skins and some people may get angry if you use "their" skin. It's safer to just download jpg skins from websites where people have said that the skin is for public use.