Use jQuery.
Make the image src point to your script, and append a querystring to the end with a unique number (using Date() / getSeconds()) every time you want to refresh the image (assuming you'll use something such as setInterval())

Using jQuery, you could use the load event to monitor when it's loaded, you might need to trigger this event each time you have refreshed the src -

$('img').load(function(){
alert('loaded');
});

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