var mess = new Array(
'<div class="maincontent" style="background-image:url(1.jpg)"></div>',
'<div class="maincontent" style="background-image:url(2.jpg)"></div>',
'<div class="maincontent" style="background-image:url(3.jpg)"></div>',
'<div class="maincontent" style="background-image:url(4.jpg)"></div>'
					 );
var max = mess.length;
var num = Math.floor((Math.random() * max));
document.writeln(mess[num]);
                    // JavaScript Document