images = new Array(5);

images[0] = "<a href = 'http://www.graphicsoptimization.com/gallery/im1.htm'><img src='http://www.graphicsoptimization.com/im/th/th1.png' alt=''></a>";
images[1] = "<a href = 'http://www.graphicsoptimization.com/gallery/im2.htm'><img src='http://www.graphicsoptimization.com/im/th/th2.png' alt=''></a>";
images[2] = "<a href = 'http://www.graphicsoptimization.com/gallery/im3.htm'><img src='http://www.graphicsoptimization.com/im/th/th3.png' alt=''></a>";
images[3] = "<a href = 'http://www.graphicsoptimization.com/gallery/im4.htm'><img src='http://www.graphicsoptimization.com/im/th/th4.png' alt=''></a>";
images[4] = "<a href = 'http://www.graphicsoptimization.com/gallery/im5.htm'><img src='http://www.graphicsoptimization.com/im/th/th5.jpg' alt=''></a>";

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

document.write(images[index]);