var bgMinTop = 240; //default
var bgHeight = 330; //default
var bgImages = [["../images/bildwelt/05.jpg", 330],
								["../images/bildwelt/06.jpg", 330],
								["../images/bildwelt/07.jpg", 330]];

function resizeWindowWithBg() {
	var id = new Date().getWeek(1)%bgImages.length;
	document.body.style.backgroundImage = "url("+bgImages[id][0]+")";
	bgHeight = bgImages[id][1];
}

function resizeWindow() {
	var p = Math.max(document.documentElement.clientHeight-bgHeight, bgMinTop);
	var el = document.getElementById("bgimage");
	el.style.backgroundPosition = "0px "+p+"px";
	window.onresize = resizeWindow;
}

Date.prototype.getWeek = function (dowOffset) {
	dowOffset = typeof(dowOffset) == 'int' ? dowOffset : 0;
	var newYear = new Date(this.getFullYear(),0,1);
	var day = newYear.getDay() - dowOffset;
	day = (day >= 0 ? day : day + 7);
	var daynum = Math.floor((this.getTime() - newYear.getTime() - (this.getTimezoneOffset()-newYear.getTimezoneOffset())*60000)/86400000) + 1;
	var weeknum;
	if(day < 4) {
		weeknum = Math.floor((daynum+day-1)/7) + 1;
		if(weeknum > 52) {
			nYear = new Date(this.getFullYear() + 1,0,1);
			nday = nYear.getDay() - dowOffset;
			nday = nday >= 0 ? nday : nday + 7;
			weeknum = nday < 4 ? 1 : 53;
		}
	} else {
		weeknum = Math.floor((daynum+day-1)/7);
	}
	return weeknum;
};