dd = new Date();
mm = dd.getMonth() + 1;

if (mm < 10) {
 mm = "0" + mm;
}

var today = mm;

season1 = 03;
end1 = 05;

season2 = 06;
end2 = 08;

season3 = 09;
end3 = 11;

season4 = 01;
season5 = 02;
season6 = 12;

if((today <= end1) && (today >= season1)){
document.write('<div id="mainmage" class="spring png">');

}
if((today <= end2) && (today >= season2)){
document.write('<div id="mainmage" class="summer png">');

}
if((today <= end3) && (today >= season3)){
document.write('<div id="mainmage" class="autumn png">');

}
if((today == season4) || (today == season5) || (today == season6)){
document.write('<div id="mainmage" class="winter png">');

}