I compiled this list for an art class, but some of the competitions are relevant to game developers and not just digital artists.
http://www.pixelprospector.com/indie-game-competitions/
Challenges/competitions/contests
https://www.artstation.com/contests
http://weeklycgchallenge.com/about/
Blizzard student art competitions: https://careers.blizzard.com/en-us/students/contests
Animation Competition: http://www.11secondclub.com/
Weekly art challenges: http://www.conceptart.org/forums/forumdisplay.php/74-Art-Community-Activities
Computer Graphics and 3D Model Design competition: https://competition.adesignaward.com/competitions/computergraphics.html
https://hum3d.com/3d-artist-competitions/
Just missed this competition too (seems may of them had end of September/beginning of October deadlines) but it seems like a big International broad appeal Digital arts competition: https://lumenprize.com/faqs/
List of competitions/contests/shows that may have a broad interpretation (fine art, illustration, cg…)
https://intercompetition.com/index.php/art
https://www.artshow.com/juriedshows/
Illustration
https://graphiccompetitions.com/illustration/nami-island-picture-book-illustration-contest-2019/