I would take it just as the rules of photography. If you take a screenshot of someone else's car/skin while in multiplayer, that is your screenshot that you have taken, thus you hold the copyrights to the screenshot. Unless you want to use said shot of skin for commercial purposes, it is your screenshot to do with what you see fit.
As long as the creator/user of the skin is in a public place (multiplayer), you have every right to take a screenshot. If the creator/user of the skin has a reasonable sense of privacy, for example he is in the bathroom or at the ATM on a cruise server, then you do not have the right to screenshot it.
If you are going to use the screenshot for commercial purposes, then you should get a model release from the creator/user of the skin.
If you use the skin yourself on your car to take a screenshot, you have violated their copyright of the graphic file and it is illegal.
It all gets very complicated when the screenshot is of someone else's skin that has used the logos of yet someone else's creation.
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