{% load thumbnail %}

<li class="slideshow-slide">
    {% if slider_thumb_id and not simple_settings %}
        <img src="{{ instance.image | thumbnail_url:slider_thumb_id  }}" alt="{{ instance.alt }}"
                {% if instance.longdesc %} title="{{ instance.longdesc }}"{% endif %} />
    {% else %}
        {% thumbnail instance.image "1920x480" crop="center" as thumb %}
        <img src="{{ thumb.url }}" width="1920" height="480" alt="{{ instance.alt }}"{% if instance.longdesc %}
             title="{{ instance.longdesc }}"{% endif %} />
    {% endif %}
</li>