Do atheists need a symbol?

Nope.
No ghosts: One more symbol for atheism
Often when atheists proselytize, one particular question comes up: do we have our own symbol? Well, we donâ..t, really. Thereâ..s a reason for that, tooâ..since atheists do not usually have a religion, there is no real reason to carry around religious symbols. Before we adopt a symbol, we should actually think long and hard whether we really want one, because having it suggests that atheism is a movement on par with actual religions. Since it isnâ..tâ..atheism is the lack of theistic beliefâ..we might not want to confuse people by creating positive content for it. This is a grave consideration, since the difference between religion and atheism is fundamental, and having positive content may actually work against our cause, e.g. in debate.