What is Riot's policy on people using their artwork?

I've wanted to start doing something on YouTube related to League of Legends for a while, am I'm working on getting something set up. I noticed that I'd need to get an avatar, background, etc, for any accounts I make about it, and I was wondering if it was possible that I could use Riot's artwork (mainly things on this page: http://na.leagueoflegends.com/en/media/art), but I'm not too sure if it would be possible because of copyrights and such. does anyone know the laws/Riot policy on this topic?