And I was wondering why. Yes, Rule 0 says not to post controversial images, but there's lots of other 9/11 jokes, so I have to wonder why those get to stay and my upload was considered uniquely worse. That was the reason I thought it was okay to post; I actually checked to see if there were any 9/11 images, and there are lots, so I was like "I guess it's okay."
The image wasn't taken down because it's based on 9/11, but because the cropping and source make clear that the poster on Tumblr wanted to make fun of the original image's creator. If you post the original, it wouldn't be taken down.