Ralph Steadman’s illustrations for the gonzo classic “Fear and Loathing in Las Vegas” are as famous as Hunter S. Thompson’s prose. The British cartoonist found a favorite subject in Richard Nixon, a man he personally loathed but loved to draw. Steadman has gone on to draw successive American presidents, including Barack Obama, in his distinctive ink and spatter style and the release of a new film about him is an opportunity to look back at his impressions of American leaders.