Ok...just got off with support and realized my error.
Do not remove or modify the first <script src=...> attribute.
Your entire HTML code should look like this:
{literal}
<script src="external/galleria/js/jquery.backstretch.min.js"></script> *Do not modify/remove this*
<script>
$.backstretch([
"public/<directory>/<image1>.jpg",
"public/<directory>/<image2>.jpg"
], {duration: 3000, fade: 750});
</script>
{/literal}
Once I added the line of HTML code back, all was right with the world again.