Hi, the skins are located in lfs/data/skins folder. Simply place your skin image there and you can load it from garage. When you press new colors it should appear in the list, but keep in mind that each skin has to be named correctly - with a 3 letter car prefix followed by an underscore, for example XFG_rane.jpg
Using a skin in a singleplayer mode, is that easy, but for multiplayer it's a bit more complicated. In order for other players to be able to see your car skin, you must first upload it to lfsworld.net.
There are plenty of skins that you can find online for various cars. Latest version of lfs lets you to even export a black&white skin template for each car, such that you can create your own skins.