//Inside of the head tags
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
 
 
//Put this wherever you want the on/offline images to be
<a src="http://twitch.tv/casy_nuf">
        <img id="nm" src="http://i.imgur.com/xNCtOBk.png"> //This image is the default, I suggest offline
</a>
 
 
//Just before the closing body tag (</body>)
<script>
    ur = 'https://api.twitch.tv/kraken/streams/casy_nuf';
    $.ajax({
        url: ur,
        dataType: 'jsonp',
        type: 'get',
        success: function(data) {
            if(data.stream === null) {
                document.getElementById("nm").src='http://i.imgur.com/xNCtOBk.png'; //offline image
            } else {
                document.getElementById("nm").src='http://i.imgur.com/JU80pML.png'; //online image
            }
        },
    });
</script>