If your add to cart form redirects to a new page then I would do it on page load of that page
you can put the jquery code inside a php conditional something like this:
<script type="text/javascript">
// <![CDATA[
<?php if(isset($_GET['image-upload']) && ($_GET['image-upload'] == 'true')) { ?>
$.gritter.add({
title: 'image upload successful',
text: 'you have successfully uploaded a new image associated with this <?php echo strtolower($i8_singular); ?>.',
image: '../../../../_img/icn/onebit/onebit_34.png',
sticky: false,
time: ''
});
<?php } ?>
// ]]>
</script>