var NUMBER_OF_IMAGES = 5;

function createClassRandomImage() {
	imgId = Math.floor(Math.random() * NUMBER_OF_IMAGES);
	document.write('<style type="text/css">\n');
	document.write(' .random_image { background-image: url("http://dl.getdropbox.com/u/2408852/blogheaders/' + imgId + '.jpg"); }\n');
	document.write('<\/style>\n');
}

createClassRandomImage();