Tim Haries told jurors he defaced the royal portrait with the word "help" to highlight what he described as the "social catastrophe" of divorced fathers being denied access to their children.
Haries, who was dressed in a Superman suit, is a member of Fathers4Justice, a group that has carried out stunts on behalf of fathers who they claim have been prevented by the courts from seeing their children.
Judge Alistair McCreath told Haries today that he had caused "deliberate and planned ... Damage to a valuable item of property on public display, carried out as a publicity exercise.