imgURL = new Array();
imgURL[0] = "img/top/main_img01.jpg"; 
imgURL[1] = "img/top/main_img02.jpg"; 
imgURL[2] = "img/top/main_img03.jpg"; 
imgURL[3] = "img/top/main_img04.jpg"; 
n = Math.floor(Math.random()*imgURL.length); 
document.write("<img src='"+imgURL[n]+"' border='0'>");