myImageCnt = 13;
myImage = new Array(
 "topimage/image1.jpg",
 "topimage/image2.jpg",
 "topimage/image3.jpg",
 "topimage/image4.jpg",
 "topimage/image5.jpg",
 "topimage/image6.jpg",
 "topimage/image7.jpg",
 "topimage/image8.jpg",
 "topimage/image9.jpg",
 "topimage/image10.jpg",
 "topimage/image11.jpg",
 "topimage/image12.jpg",
 "topimage/image13.jpg"
);
 
myRnd = Math.floor(Math.random()*myImageCnt);
document.write("<img src='",myImage[myRnd],"' border='0' >");