function image() {
};

image = new image();
number = 0;

// imageArray
image[number++] = "<img src='img/1.jpg'>"
image[number++] = "<img src='img/2.jpg'>"
image[number++] = "<img src='img/3.jpg'>"
image[number++] = "<img src='img/4.jpg'>"
image[number++] = "<img src='img/5.jpg'>"
image[number++] = "<img src='img/6.jpg'>"
image[number++] = "<img src='img/7.jpg'>"
image[number++] = "<img src='img/8.jpg'>"
image[number++] = "<img src='img/9.jpg'>"
image[number++] = "<img src='img/10.jpg'>"
image[number++] = "<img src='img/11.jpg'>"
image[number++] = "<img src='img/12.jpg'>"
image[number++] = "<img src='img/13.jpg'>"
image[number++] = "<img src='img/15.jpg'>"
image[number++] = "<img src='img/16.jpg'>"
image[number++] = "<img src='img/17.jpg'>"
image[number++] = "<img src='img/18.jpg'>"
// keep adding items here...

increment = Math.floor(Math.random() * number);

document.write(image[increment]);