From http://ecommerce.shopify.com/c/ecommerce-design/t/pull-first-image-from-each-tag-so-subcategories-have-an-image-associated-with-link-to-tag-149029

new_gist_file

<div class="clearfix image-subcats">
      <!-- Begin image subcats -->
        {% if settings.collection_tags %}
          {% capture col_url%}{% if collection.url.size == 0 %}/collections/all{%else%}{{collection.url}}{%endif%}{%endcapture%}
          {% if collection.all_tags.size > 0 %}
          	{% for tag in collection.all_tags %}
          	<div class="product span3">
          	  <div class="image">
                <a href="{{col_url}}/{{tag | handle}}">
                  {% capture file_name %}{{ collection.handle }}-{{ tag | handle }}.jpg{% endcapture %}
                  {% capture alt_attr %}{{ collection.title | escape }}-{{tag | handle}}{% endcapture %}
                  {{ file_name | asset_url | img_tag: alt_attr }}
                </a>
              </div>
              <h3>{{ tag | link_to_tag: tag }}</h3>
            </div>
        	{% endfor %}
          {% endif %}
        {% endif %}
      <!-- End image subcats --> 
  </div>