Most of the time, skin concepts need to get an overwhelming amount of support from the community before they officially "become a thing." Yeah, surprise party fiddlesticks became a thing, but only after most of the community and several Rioters loved the idea, and then I'm pretty sure they handled all the art and modeling in-house after that. Also, if you don't pursue a claim that they're using your copyrighted material, no one's going to take any legal action. If you're looking to get paid for it, then you're probably doing skin concepts for the wrong reason. Most people just draw them in their spare time to (hopefully) make the game better if the community is behind it, not make money for their ideas.