images = new Array(4);

images[0] = "<img id='randomimage-6' class='leftcol' src='images/6a.jpg' width='210' height='120' alt='Underwater' />";	

images[1] = "<img id='randomimage-6' class='leftcol' src='images/6b.jpg' width='210' height='120' alt='Underwater' />";

images[2] = "<img id='randomimage-6' class='leftcol' src='images/6c.jpg' width='210' height='120' alt='Underwater' />";

images[3] = "<img id='randomimage-6' class='leftcol' src='images/6d.jpg' width='210' height='120' alt='Underwater' />";

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

document.write(images[index]);