function image() {
};

image = new image();
number = 0;

// imageArray
image[number++] = "<img src='images/portfolio-show1.jpg' border='0' alt='slide 001'>"
image[number++] = "<img src='images/portfolio-show2.jpg' border='0' alt='slide 002'>"
image[number++] = "<img src='images/portfolio-show3.jpg' border='0' alt='slide 003'>"
image[number++] = "<img src='images/portfolio-show4.jpg' border='0' alt='slide 004'>"
image[number++] = "<img src='images/portfolio-show5.jpg' border='0' alt='slide 005'>"
// keep adding items here...

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

document.write(image[increment]);