function changeImg(imgNumber) {
var myImages = ["http://www.marstudiosites1.com/ess/images/bgs/bg1.jpg",
				"http://www.marstudiosites1.com/ess/images/bgs/bg2.jpg",
				"http://www.marstudiosites1.com/ess/images/bgs/bg3.jpg",
				"http://www.marstudiosites1.com/ess/images/bgs/bg4.jpg",
				"http://www.marstudiosites1.com/ess/images/bgs/bg5.jpg",
				"http://www.marstudiosites1.com/ess/images/bgs/bg6.jpg",
				"http://www.marstudiosites1.com/ess/images/bgs/bg7.jpg",
				];
var imgShown = document.getElementById('bigbg').style.backgroundImage;
var newImgNumber =Math.floor(Math.random()*myImages.length);
document.getElementById('bigbg').style.backgroundImage = 'url('+myImages[newImgNumber]+')';
}
window.onload=changeImg;