function image() {
};

image = new image();
number = 0;

// imageArray
image[number++] = "<img src='imgs/img01.jpg' border='0'>"
image[number++] = "<img src='imgs/img02_old.jpg' border='0'>"
image[number++] = "<img src='imgs/img03.jpg' border='0'>"
image[number++] = "<img src='imgs/img04.jpg' border='0'>"
image[number++] = "<img src='imgs/img05.jpg' border='0'>"

// keep adding items here...

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

document.write(image[increment]);