Quora has some links in a similarly worded question that might provide further insight https://www.quora.com/What-is-the-best-website-for-free-game-art-assets-and-game-development-resources
Also, a Github repo with resources for indie game developers that includes graphics: https://github.com/ellisonleao/magictools
And another: https://github.com/DaRaFF/jsgamewiki