Thank you for the help, guys. I have two different types of posters in mind, now. The original posters (Which do not all have to be the same, but I would rather not have them offensive. This allows us to to recruit a base following of passive atheists) and later posters (which would be more for encouraging debates and getting the more aggressive atheists who do not feel like going to passive meetings).
In the former category, I would like to have a picture with an attention grabbing caption. For pictures, I like either the brain with a lightbulb inside or a picture of Earth as seen from space. For the captions, I like:
You can be Good without God.
Atheist? So are we.
Ready to think outside
Looking for answers that don't
involve the supernatural?
(The atheist) A
For the latter posters, I like the amputee idea and the FSM one. I think that could provoke some discussion.
Sorry if I did not take up your suggestion, it was appreciated nonetheless. I am trying not to turn 95% of the school against us and reinforce the idea that atheists are always the bad guys. It is a fine line to walk (and admittedly extremely entertaining to do so).