When any advertisement has been exhibited, fixed or retained upon or over any land, building, wall, boarding, post or structure or upon or in any vehicle or displayed to public view from a public street or public place in contravention of the provisions of this Act or any bye-laws made thereunder, it shall be presumed, unless and until contrary is proved, that the contravention had been committed by the person or the persons on whose behalf the advertisement purports to be or the agents of such person or persons.