var images = new Array();

images[0] = "media/f1.jpg"; 
images[1] = "media/f2.jpg";
images[2] = "media/f3.jpg";
images[3] = "media/f4.jpg"; 
images[4] = "media/f5.jpg";
images[5] = "media/f6.jpg";
images[6] = "media/f7.jpg"; 
images[7] = "media/f8.jpg";
images[8] = "media/f9.jpg";
images[9] = "media/f10.jpg"; 
images[10] = "media/f11.jpg";
images[11] = "media/f12.jpg";

function ShowImg() {
var number = images.length;
var increment = Math.floor(Math.random() * number);
var strTemp ='<img src="' + images[increment] + '" border="0">';
document.write(strTemp);
}