var head = document.getElementsByTagName('head')[0];
var style = document.createElement('style');
var today = new Date();

switch(today.getMonth())
{
	case 0: //January
		var imageUrl = "http://www.reading-rspb.org.uk/images/headers/index_header.jpg";
		break;
	case 1: //February
		var imageUrl = "http://www.reading-rspb.org.uk/images/headers/index_header.jpg";
		break;
	case 2: //March
		var imageUrl = "http://www.reading-rspb.org.uk/images/headers/index_header.jpg";
		break;
	case 3: //April
		var imageUrl = "http://www.reading-rspb.org.uk/images/headers/index_header.jpg";
		break;
	case 4: //May
		var imageUrl = "http://www.reading-rspb.org.uk/images/headers/index_header.jpg";
		break;
	case 5: //June
		var imageUrl = "http://www.reading-rspb.org.uk/images/headers/index_header.jpg";
		break;
	case 6: //July
		var imageUrl = "http://www.reading-rspb.org.uk/images/headers/index_header.jpg";
		break;
	case 7: //August
		var imageUrl = "http://www.reading-rspb.org.uk/images/headers/index_header.jpg";
		break;
	case 8: //September
		var imageUrl = "http://www.reading-rspb.org.uk/images/headers/index_header.jpg";
		break;
	case 9: //October
		var imageUrl = "http://www.reading-rspb.org.uk/images/headers/index_header.jpg";
		break;
	case 10: //November
		var imageUrl = "http://www.reading-rspb.org.uk/images/headers/index_header.jpg";
		break;
	case 11: //December
		var imageUrl = "http://www.reading-rspb.org.uk/images/headers/index_header.jpg";
		break;
}

rules = document.createTextNode('#image_header { background: transparent url("'+imageUrl+'") no-repeat top left; }'); 

style.type = 'text/css'; 
if(style.styleSheet) 
    style.styleSheet.cssText = rules.nodeValue; 
else style.appendChild(rules); 
head.appendChild(style);