They have 4 splash artists, 4 concept artists and 4 people who basically make the splashes look better (enhancers and animator who make some of the stuff in the splash art to the game) (old information, might have changed since then)
In addition, they have many quest artists:
Suke Su used to be a quest artist but was recently employed by Riot Games I think (just an example).
Some of his works:
http://fc03.deviantart.net/fs70/f/2014/313/b/4/kayle_new_splash_by_su_ke-d85w02l.jpg
http://fc00.deviantart.net/fs70/f/2014/313/8/4/ezreal_splash_base_rework_by_su_ke-d85w2uj.jpg
In addition, they have many quest artists:
Suke Su used to be a quest artist but was recently employed by Riot Games I think (just an example).
Some of his works:
http://fc03.deviantart.net/fs70/f/2014/313/b/4/kayle_new_splash_by_su_ke-d85w02l.jpg
http://fc00.deviantart.net/fs70/f/2014/313/8/4/ezreal_splash_base_rework_by_su_ke-d85w2uj.jpg

Maintained wrote:
You might have just seen the Chinese splash art for Hecarim.
Well, I haven't noticed it before but it's already like two years old, my bad.
I thought it might be the same treatement similar to splash arts appearing first on the Asian and then on the western servers.

Thanks to Natuhlee for this sig!
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Honestly, most of the splash arts are fine now. There are also a few new ones I personally dislike such as the
By the way, isn't a new