var image = new Array()

image[0] = 'images/main/01.jpg'
image[1] = 'images/main/02.jpg'
image[2] = 'images/main/03.jpg'
image[3] = 'images/main/04.jpg'
image[4] = 'images/main/05.jpg'
image[5] = 'images/main/06.jpg'
image[6] = 'images/main/07.jpg'
image[7] = 'images/main/08.jpg'
image[8] = 'images/main/09.jpg'
image[9] = 'images/main/10.jpg'
image[10] = 'images/main/11.jpg'

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