$(document).ready(function(){

	Date.prototype.getWeek = function() 
	{
		var onejan = new Date(this.getFullYear(),0,1);
		return Math.ceil((((this - onejan) / 86400000) + onejan.getDay())/7);
	}

	var today = new Date();
	var weekno = today.getWeek();

	var theImages = new Array();

	theImages[0] = 'one';
	theImages[1] = 'two';
	theImages[2] = 'three';
	theImages[3] = 'four';

	document.getElementById('something').className = theImages[weekno % 4];

});