You need to save the skins you made in your lfs/data/skins folder. Make sure you named them identically to the ones you uploaded to lfsworld. Select the skin you want to use in your car setup screen, under "Info"
Well, it should be wherever you installed LFS to. The "lfs" part is just an indication. Look up your main LFS directory, and there is a data folder in it, and inside that one is a skins folder.