here's another~
https://www.asianfanfics.com/story/view/1306987/

I do know of these two:
http://www.asianfanfics.com/story/view/890302/sweet-lies-graphic-contest-r1-open-contest-graphics-karma-postercontest-prizes-graphiccontest-graphicchallenge
http://www.asianfanfics.com/story/view/1316528/precise-graphic-contest-accepting-graphics-entries-grand-opening-challenge-contest-graphic-graphics-kpop-hiring-layouts-layout-advertisement-portfolio-graphicdesign-vixx-bts-graphiccontest-nct-desginers

These are the ones that are open, but if you subscribe to the "graphiccontest" tag feed (http://www.asianfanfics.com/browse/tag/graphiccontest/L), you would get updates if there are more.

Good luck!