image-preview.html

<img id="preview" style="display: none; max-width: 200px" src="">

<script src="https://code.jquery.com/jquery-1.11.3.min.js"></script>
<script>
    $(document).ready(function() {
        $("#photo_url").on("blur", function(event) {
            var photo_url = $(this).val();
            var preview = $("#preview");
            if (photo_url) {
                preview.attr("src", photo_url)
                preview.show();
            } else {
                preview.hide();
            }
        });
    });
</script>