I think its close, but not quite it. Its too vague. You need to tie in a purpose, deed, or act. Like I mentioned in my suggestion, have the model doing something that most people don't think atheists do, like volunteer in community organizations, or sit on two sides of a political issue, or respect other opinions, or whatever. Using the group's name, "THINK ATHEIST," should be directly tied into something that demonstrates atheism, but not in a shocking or offensive way. Like if this billboard said "I THINK... Science is for school and religion is for church. I THINK... ATHEIST." Then smaller, but still visible, put the website, "THINKATHEIST.COM."
Has anyone checked to see if THINKATHEIST.ORG (vs. .COM) is available? A non-profit suffix might be more appealling to skeptics of our message. Just a thought! - PIPER
As much as I like that picture, I think that it might be a little too artistic for the general public. It's a beautiful shot, but most people will not identify with it.
What about a composite picture of lots of smiling faces of site members? Kind of like the collage idea, but I don't even think that it would have to necessarily form a larger image. Maybe just around twenty or so headshots, and a tagline that says something along the lines of, "Do you think atheist? We do! Come visit us at www.ThinkAtheist.com"
I think that it would be good to show a bunch of normal, happy, smiling faces that people could identify with; if we got a varied group of ages, races, and genders we would ensure that nearly every viewer would find a someone on the billboard that they identified with.
(Edit: I definitely don't mean to imply that the model above does not look normal or happy. I really do love that picture, and I would definitely include her headshot in the mix. But I just don't think that by itself this picture would have mass appeal for your average person.)