var bg1         = new Image();
var bg2         = new Image();
var bg3         = new Image();
var bg4         = new Image();

bg1.src         = '/images/backgrounds/runs.jpg';
bg2.src         = '/images/backgrounds/kids.jpg';
bg3.src         = '/images/backgrounds/bike2.jpg';
bg4.src         = '/images/backgrounds/festival.jpg';

function setBackgroundImage(thisImage) {

  var pageBackground  = document.getElementById('background-image');

  if( typeof(pageBackground) == 'object' ) {

    switch(thisImage) {
    case 1: pageBackground.src = bg1.src; break;
    case 2: pageBackground.src = bg2.src; break;
    case 3: pageBackground.src = bg3.src; break;
    case 4: pageBackground.src = bg4.src; break;
    }

  }

}