You need to include jQuery library file to use jQuery methods.
In the below sample a jquery cdn version is added. Also to slide down an image it has to be hidden first
<!DOCTYPE html>
<html>
<head>
<title>
</title>
<style>
</style>
<script type="text/javascript" src="http://code.jquery.com/jquery-1.10.2.min.js"></script>
<script type="text/javascript">
$(document).ready(function() {
$('img').slideDown('slow');
});
</script>
</head>
<body>
<img src="images\sjmak-music-logo.jpeg" style="display: none"/>
</body>
</html>
Demo: Fiddle