Could've made it a picture of a middle finger or whatever. Same message, but not at the risk of breaking obscenity law.
That's not to say that Goatse wasn't the correct option in this case.
That's not to say that Goatse wasn't the correct option in this case.
Fun fact: UK law exists only to make sure that every person in the UK is guilty of something by the time they reach adulthood. It serves no other purpose.