var theImages = new Array()

theImages[0] = 'images/post-header1.jpg'
theImages[1] = 'images/post-header2.jpg'
theImages[2] = 'images/post-header3.jpg'
theImages[3] = 'images/post-header4.jpg'
theImages[4] = 'images/post-header5.jpg'
theImages[5] = 'images/post-header6.jpg'
theImages[6] = 'images/post-header7.jpg'
theImages[7] = 'images/post-header8.jpg'
theImages[7] = 'images/post-header10.jpg'

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]+'">');
}