function $(e) { return document.getElementById(e); }
function changeBg(){
	var no = Math.floor(Math.random() * 3) ;
	
	var imgArr = new Array;
	imgArr[0] = 'main_image.jpg';
	imgArr[1] = 'main_image02.jpg';
	imgArr[2] = 'main_image03.jpg';
	$('mainImg').src = "images/" + imgArr[no];
}
window.onload = function(){
	changeBg();
}