It is such an obvious thought, I suppose it has already been suggested, but - -
A picture of the World Trade Center pre Sept. 11, 2001 with the phrase "Imagine No Religion - Think. Atheist."
Wow! Texas Governor Rick Perry declared a month of prayer for Texans to pray for rain during their extreme drought this year. Guess what? It didn't work. How about a Texas billboard which drills into that??? Prayer didn't work? Try Science! (and send Perry packing)