Shock value gets a lot of attention but often it's fuel for those against us. Perhaps something simple. A man or woman looking in deep thought. I think black and white would look classy with a quote such as. "The important thing is not stop questioning" or "Questioning is the door of Knowledge"
There are a lot of questioning people out there and maybe they want to dip a toe in and see how the water feels but maybe they grew up in the church or unsure. I think being settle, welcoming, and understanding would be more appealing.
Another idea is having a picture of a family, a normal everyday family and it reads. "Atheism isn't what you think"
And give a web address to both inviting for people to explore for themselves. People get defensive when you are trying to say that they are wrong, just lay down the road for people to make that decision for themselves. Just a few ideas.