i already said that riot used their own artists- but they cannot get away with obvious stuff like that unless they got permission from Nintendo which most likely involves paying them a sht ton of money just to have Nintendo theme related skins now - before when Riot wasn't that big back in beta and season 1 no one cared so Riot could do whatever the hell they wanted to - popularity and growth comes more responsibility and tighter rules
as far as fanart goes, you cannot sue someone for fanart unless you stole someone else's art otherwise as long as you are the artist that DIRECTLY made the work and labeled it as fanart and stated what the art is from then it is fine and you won't get in trouble [due note that you cannot make money from fanart, if you do not offer your directly made fanart as 100% free then that is where you get sued for it]
i suggest you give that link i posted earlier a long read, it has all of the legality of fan art and fan fiction printed out fairly clearly. When you make a picture of a copyrighted character like say rengar, and exhibit it by posting it online , whether you are making money on it or not, riot has the right to destroy your art and sue you if they so chose and can and will win in court. it has been done too many times already. But Riot allows us to do fan art so no worries there.
There is a lot of myth that clouds the truth of fan art in regards to what you can and cannot be sued for.
Perhaps you’ve heard some of the following myths:
One-of-a-kind, original drawings and paintings are legal.
Since everyone does it, copyright holders must not care.
If I only sell fan art at conventions, and not online or in stores, it is okay.
If I’m not making a profit from my fan art, it is legal to draw someone else’s characters.
Fan art and copyright infringement might seem complicated, but really it isn’t:
Do you own the character in question?
Yes? Do what you please!
No? You have no legal right to profit from work featuring characters without permission from copyright holder.
No? You have almost no rights to create such works not for a profit, either.
Don't believe me, watch this video at San Diego comic con "Fan Art Law at Comic-Con" by deviant art. Feel free to skip the first 6 minutes. https://www.youtube.com/watch?v=xKBsTUjd910
Will you always get sued for fan art or fanfic? No, but the fact remains that you can be. It all depends on the company/artists/writer/creator/ip in question.