zola/docs/templates/shortcodes/gallery.html

{% for asset in page.assets -%}
  {%- if asset is matching("[.](jpg|png)$") -%}
    <a href="{{ get_url(path=asset) | safe }}" target="_blank">
      <img src="{{ resize_image(path=asset, width=240, height=180, op="fill", format="auto") | safe }}" />
    </a>
  {%- endif %}
{%- endfor %}