I did a similar thing, although the racer had made the mistake of putting his picture in his lfs world profile, so I downloaded it, turned him into a transvestite, and put some interesting comments all over the skin.
But, as there is no way to prevent it, that means it will get done.
Public servers are stocked with people who can only get attention
through being obnoxious, and they are usually more interested in
aggravating you than having your praticular skin.
Post you skin on the forum when you've developed it, and place hidden
indications that you are the creator - make a nearly transparent reference
to 'skin by such and such' some where out of the way. Incorporate your name in a way that can't easily be erased i.e. across one of the major elements in colors that blend in, again, in a fairly transparent manor.
Make your first display of it around people you know well - none of this will stop theft, but might make you feel better knowing that you don't have get in to ( desired ) argument of 'nu-huh, you stole it from ME'.
In the end, take comfort in knowing you achieved what you set out to do,
and also, know that the person stealing it is probably not skilled at other
aspects of LFS, so go out and beat the hell out of them on the track
at every possiblity.