jQM Template

HTML

<link rel="stylesheet" href="http://jeromeetienne.github.com/jquery-mobile-960/css/jquery-mobile-fluid960.css">
<script src="http://timeago.yarp.com/jquery.timeago.js"></script>
<script src="http://imakewebthings.github.com/jquery-waypoints/waypoints.min.js"></script>

JavaScript

$.UrlExists = function(url) {
	var http = new XMLHttpRequest();
    http.open('HEAD', url, false);
    http.send();
    return http.status!=404;
}

if($.UrlExists("http://google.com/")){
	alert("img1.jpg");
}else{
	alert("img2.jpg");
}