function rnd(a,b) {
	return a + Math.floor(Math.random()*(b-a+1));
}

function randomImage(num) {
	var ele = $('wrapper');
	var random = rnd(1,num);
	var myclass = 'image'+random;
	Element.addClassName(ele,myclass);
}

//Event.observe(window, 'load', randomImage(6), false);

window.onload = function() {randomImage(6);};