var theImages = new Array(); 

theImages[0] = 'banners/banner_1.gif';
theImages[1] = 'banners/banner_2.gif';
theImages[2] = 'banners/banner_3.gif';
theImages[3] = 'banners/banner_4.gif';
theImages[4] = 'banners/banner_5.gif';

var j = 0;
var p = theImages.length;
var preBuffer = new Array();
for (i = 0; i < p; i++){
   preBuffer[i] = new Image();
   preBuffer[i].src = theImages[i];
};
var whichImage = Math.round(Math.random()*(p-1));
function showImage(){
document.write('<img src="'+theImages[whichImage]+'">');
}