You have to understand the attitude
Yes, the joke portrays rebuilding the WTC to look like a popular obscene hand gesture. The gesture is pointed at those terrorists who think they can make us cower in our homes. New Yorkers are world (in)famous for their attitude. We know who are friends are and truly appreciate the sentiments expressed in the past week. We also refuse to back down to those who seek to do us harm. Yeah, that's copping at attitude. You gotta problem with that? ;}