var bg_alert = ['main_wm1.jpg','main_wm2.jpg','main_wm3.jpg'];
var aImages = [];
for (var i = 0, len = bg_alert.length; i < len; i++) {
        aImages[i] = new Image();
        aImages[i].src = bg_alert[i];
}
function change_bg_alert() {
	a = parseInt($('#alert_bg').attr('bg'));
	b = a + 1;
	if (b > bg_alert.length) b = 1;
	$('#alert_bg').attr('bg',b);
	$('#alert_bg').css('background-image',$('#alert_bg').css('background-image').replace(bg_alert[a-1],bg_alert[b-1]));
//	alert('url(http://upload/alert/'+bg_alert[a-1]+')');
}

setInterval("change_bg_alert();",3000);