switch(new Date().getMonth())
{
  case 0: case 1: case 2: 
		img = "backgrounds/winter.jpg";
	break;
  case 3: case 4: 
		img = "backgrounds/spring.jpg";
	break;
  case 5: case 6: case 7: 
		img = "backgrounds/summer.jpg";
	break;	
  case 8: case 9: case 10:
		img = "backgrounds/autumn.jpg";
	break;
   case 11: case 12:
		img = "backgrounds/winter.jpg";
	break;	
  default:
		img = "backgrounds/summer.jpg";
}

document.body.style.backgroundImage = "url("+[img]+")";