function image() {
};

image = new image();
number = 0;

// imageArray
image[number++] = "<img src='image/i1.jpg'>"
image[number++] = "<img src='image/i2.jpg'>"
image[number++] = "<img src='image/i3.jpg'>"
image[number++] = "<img src='image/i4.jpg'>"
image[number++] = "<img src='image/i5.jpg'>"
image[number++] = "<img src='image/i6.jpg'>"
image[number++] = "<img src='image/i7.jpg'>"
image[number++] = "<img src='image/i8.jpg'>"
image[number++] = "<img src='image/i9.jpg'>"
// keep adding items here...

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

document.write(image[increment]);