var chang_img = {
				get_node: function(id){
					return document.getElementById(id);
				},
				prepare_img: function(){
					return ["images/banner.jpg","images/banner01.jpg","images/bannerr_1.jpg","images/bannerr_3.jpg","images/bannerr_4.jpg","images/bannerr_5.jpg","images/bannerr_6.jpg"];
				},
				bg: function(id){
					var img_num = this.prepare_img().length;
					var i = Math.floor(img_num * Math.random(1));
					this.get_node(id).style.backgroundImage = 'url(' + this.prepare_img()[i] +')';
				},
				s:function(){
					alert(this.prepare_img[1]);
				}
			}
			window.onload = function(){
				chang_img.bg("test");
			}