Basically, i downloaded a public skin, putted it in my skins folder(with the same name). And were going to drive some laps online. I received no errors and another player told me that i have a nice skin. And ofcours i didn`t upload it to LFSWorld.
If your skin has the same name as a skin on LFSWorld, other people will download the LFSWorld one and see that. If your skin differs from the LFSWorld one (but has the same name) you will see a different skin to what they see.