gistfile1.html

<img src="logo.svg" width="200" height="300">

<img src="logo.svg" data-png-fallback="logo.png"
width="200" height="300">

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/modernizr/2.6.2/modernizr.min.js"></script>
<script>
$(function(){
  if(!Modernizr.svg) {
    $('[data-png-fallback]').each(function() {
      $(this).attr('src', $(this).data('png-fallback'));
    });
  }
});
</script>