Alert boxes in JavaScript can only display pure text. You could use a JavaScript library like jQuery to display a modal instead?
This might be useful: http://jqueryui.com/dialog/
You can do it like this:
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title>jQuery UI Dialog - Default functionality</title>
<link rel="stylesheet" href="http://code.jquery.com/ui/1.10.3/themes/smoothness/jquery-ui.css" />
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
<script src="http://code.jquery.com/ui/1.10.3/jquery-ui.js"></script>
<script>
body {
font-family: "Trebuchet MS", "Helvetica", "Arial", "Verdana", "sans-serif";
font-size: 62.5%;
}
</script>
<script>
$(function() {
$( "#dialog" ).dialog();
});
</script>
</head>
<body>
<div id="dialog" title="Basic dialog">
<p>Image:</p>
<img src="http://placehold.it/50x50" alt="Placeholder Image" />
</div>
</body>
</html>