Adding new custom social genericons in header

Testing out the Ryu theme again…anyone know of a way to add custom genericons (the social icons in the top dropdown feature), or perhaps find the existing images of the available channels (Facebook, Github, LinkedIn, etc) to edit?

I probably won’t use half of them, so, for instance, using the “dribbble” link but with a custom icon to another site like Behance would be fine.