i dont see the problem considering there are links in the post to where you submit it and a link to the source at the bottom
so even if they thought it was a mobafire contest and still tried it they would still be submitting their art to riot
so even if they thought it was a mobafire contest and still tried it they would still be submitting their art to riot

Thanks to TheNamelessBard for the signature
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To celebrate legendary battles and equally epic fanart, we're launching the League of Legends Fanart Face Off contest! We challenge you to create an original piece of art—drawn, painted, digital, or in any other two-dimensional format—that depicts two or more champions in the midst of battle.
The top 10 winners will receive a bundle of skins and RP:
Contest rules:
The contest submission period starts today and ends on September 17 at 12:59 PM PST. On September 26, we'll announce the winners! As all of you know, we prefer to have the community judge these types of contests. However, the tech we're using to run the contest is still in beta, so we'll need to keep voting internal this time around.
If you're looking for more info, check out the official rules. When you're ready to submit your masterpiece, head over here!
Banner art by: Jepoykalboh
Source: http://na.leagueoflegends.com/en/news/community/contests/face-our-fanart-contest