The designer should know enough to you visual symbols that have real-world equivalents to convey the meaning of the button visually. Envelopes for mail. phones for contact button. Thumbs up to like, thumbs down to dislike.
This is my first time learning about the open source project (never been on hacker news before). Can anyone provide a link to what you would call a typical open source project in need of a designer. I'm terrified of working with you guys but am starting to feel like maybe I should get over that and lend a hand.