var theImages = new Array()
// сюда добавляем картинки...
theImages[0] = 'myy/3.jpg'
theImages[1] = 'myy/5.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 showImagess(){
   document.write('<img class=im src="'+theImages[whichImage]+'">');
}