Date.prototype.getWeek = function (dowOffset) {
/*getWeek() was developed by Nick Baicoianu at MeanFreePath: http://www.meanfreepath.com */

dowOffset = typeof(dowOffset) == 'int' ? dowOffset : 0; //default dowOffset to zero
var newYear = new Date(this.getFullYear(),0,1);
var day = newYear.getDay() - dowOffset; //the day of week the year begins on
day = (day >= 0 ? day : day + 7);
var daynum = Math.floor((this.getTime() - newYear.getTime() -
(this.getTimezoneOffset()-newYear.getTimezoneOffset())*60000)/86400000) + 1;
var weeknum;
//if the year starts before the middle of a week
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;
/*if the next year starts before the middle of
the week, it is week #1 of that year*/
weeknum = nday < 4 ? 1 : 53;
}
}
else {
weeknum = Math.floor((daynum+day-1)/7);
}
return weeknum;
};

bildarray = new Array(
"images/titel1.jpg",
"images/titel2.jpg",
"images/titel3.jpg",
"images/titel4.jpg",
"images/titel5.jpg",
"images/titel6.jpg",
"images/titel7.jpg",
"images/titel8.jpg",
"images/titel9.jpg",
"images/titel10.jpg",
"images/titel11.jpg",
"images/titel12.jpg",
"images/titel13.jpg",
"images/titel14.jpg",
"images/titel15.jpg",
"images/titel16.jpg",
"images/titel17.jpg",
"images/titel18.jpg",
"images/titel19.jpg",
"images/titel20.jpg",
"images/titel21.jpg",
"images/titel22.jpg",
"images/titel23.jpg",
"images/titel24.jpg",
"images/titel25.jpg",
"images/titel26.jpg",
"images/titel27.jpg",
"images/titel28.jpg",
"images/titel29.jpg",
"images/titel30.jpg",
"images/titel31.jpg",
"images/titel32.jpg",
"images/titel33.jpg",
"images/titel34.jpg",
"images/titel35.jpg",
"images/titel36.jpg",
"images/titel37.jpg",
"images/titel38.jpg",
"images/titel39.jpg",
"images/titel40.jpg",
"images/titel41.jpg",
"images/titel42.jpg",
"images/titel43.jpg",
"images/titel44.jpg",
"images/titel45.jpg",
"images/titel46.jpg",
"images/titel47.jpg",
"images/titel48.jpg",
"images/titel49.jpg",
"images/titel50.jpg",
"images/titel51.jpg",
"images/titel52.jpg",
"images/titel53.jpg");

var mydate = new Date();
var weeknumber = mydate.getWeek();

function MonatsGrafik()
{
document.getElementById("Monatsbild").src = bildarray[weeknumber];
}

setTimeout('MonatsGrafik()',1);
document.write("<img src=http://www.avifauna-thueringen.de/images/titel"+weeknumber+".jpg>");