var imgMax  = 5;
var imgRand = Math.floor(Math.random() * imgMax);

if(imgRand == 0)       document.write('<img src="images/top_image1.jpg" width="450" height="80" alt="IMAGE" title="">');
else if(imgRand == 1)  document.write('<img src="images/top_image2.jpg" width="450" height="80" alt="IMAGE" title="">');
else if(imgRand == 2)  document.write('<img src="images/top_image3.jpg" width="450" height="80" alt="IMAGE" title="">');
else if(imgRand == 3)  document.write('<img src="images/top_image4.jpg" width="450" height="80" alt="IMAGE" title="">');
else if(imgRand == 4)  document.write('<img src="images/top_image5.jpg" width="450" height="80" alt="IMAGE" title="">');