function image() {
};

image = new image();
number = 0;

// imageArray
image[number++] = "<img src='images/malehero.jpg' border='0' height='140' align='right' hspace='5' alt='Bookmobile'>"
image[number++] = "<img src='images/femalehero.jpg' border='0' height='140' align='right' hspace='5' alt='Bookmobile'>"

// keep adding items here...

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

document.write(image[increment]);