function rndNum(c) {return Math.floor(Math.random()*c+1)-1;}

function randomImage(){
	var _imageGroup = [
		'images/random-image/switch-image1.jpg',
		'images/random-image/switch-image2.jpg',
		'images/random-image/switch-image3.jpg'
	];

	var _image = document.getElementById('randomImage');
	if(_image){
		var src = _imageGroup[rndNum(_imageGroup.length)];
		_image.src = src;
	}
}
if (window.addEventListener) window.addEventListener("load", randomImage, false);
else if (window.attachEvent) window.attachEvent("onload", randomImage);