function setImage(file_names) {
    var max_img = file_names.length;
    var img_num = Math.floor(Math.random() * max_img);

    document.write('<sty'+'le type="text/css">');
    document.write('#mainImage p { background-image: url('+file_names[img_num]+');}');
    document.write('</st'+'yle>');
}