From a design perspective, I don't like the literal representation of the turds--especially the one which is still coming out of the dogs butt. I think it would have been better if just a single turd mound was used, or just a sign with a turd mound instead of a dog.
But the main flaw is that you'd have a hard time stopping a dog from actually pooping. They just want you to clean it up. It's as if the sign was meant to be read by dogs and not humans.