images = new Array(2);
images[0]='<img src="images/hardware_mainImage_1.jpg" alt="" width="632" height="354" border="0" usemap="#mainImage1">';
images[1]='<img src="images/hardware_mainImage_2.jpg" alt="" width="632" height="354" border="0" usemap="#mainImage2">';

index=Math.floor(Math.random() * images.length);

document.write(images[index]);