var bg = new Array();

bg[bg.length]='https://img10.hostingpics.net/pics/41296Banniere_1.png';
bg[bg.length]='https://img10.hostingpics.net/pics/798859Banniere_2.png';
bg[bg.length]='https://img10.hostingpics.net/pics/855905Banniere_3.png';

document.body.style.backgroundImage='url('+bg[Math.floor(Math.random()*bg.length)]+')';