function roll_over(img_name, img_src) {
	document[img_name].src = img_src;
}
var url = "index.php";
function antiCache (url){
	//sorteio um valor número de 0.0000000 a 1
	var sorteio = Math.random();	
	//verifica se a url já possui variável via GET
	//ou seja, possui '?'
	if (url.indexOf("?")!=-1) {
		url += "&_="+sorteio;
	} else {
		url += "?_="+sorteio;
	}
	//retorna a url com a valor randomico
	return url;
}
<!-- hide from none JavaScript Browsers
Image0 = new Image()
Image0.src = "images/background_x.gif"

Image5 = new Image()
Image5.src = "images/background_y.gif"

Image1= new Image(170,87)
Image1.src = "images/menu_cond_03_mini.gif"

Image2 = new Image(170,87)
Image2.src = "images/menu_cond_06_mini.gif"

Image3 = new Image(170,87)
Image3.src = "images/menu_cond_08_mini.gif"

Image4 = new Image(170,87)
Image4.src = "images/menu_cond_10_mini.gif"