images = new Array(2);

images[0] = "<img src='http://www.mingpaofun.com/van/student_reporter/image/indexbg/s_reporter_bg_index_04B.jpg'>";

images[1] = "<img src='http://www.mingpaofun.com/van/student_reporter/image/indexbg/s_reporter_bg_index_04G.jpg'>";

index = Math.floor(Math.random() * images.length);

document.write(images[index]);