How can graphic designers contribute to Open Source projects?

I want to participate in Open Source projects, but I cannot code (that well. Or.. at all).

I would love to contribute to GitHub projects, but as a designer I'm not sure what's the best way to do this (I mention GitHub because it makes it really easy to find stuff which is already happening, but any platform would apply).

What's the place of a graphic designer in the Open Source world, or what are some good practises when deciding to join a project of these characteristics?