var randomnumber = Math.floor(Math.random()*6)
var imgfstPart 	 = "banner";
var imglstPart 	 = ".gif";

document.write('<a href="http://www.layoutgalaxy.com/index.php4?reftypid=affiliate&refid=121&linkid=2&fun=1" target="_blank"><img src="images/'+imgfstPart+randomnumber+imglstPart+'" border="0"></a>');