admin#show.html.erb

<!-- 商品圖片 -->
<div class="row">
  <% if @product_images.present? %>
  <!-- 第一張圖為 medium 尺寸大圖 -->
  <div class="col-md-12">
    <%= image_tag(@product_images.first.image.thumb.url, class: 'main-image') %>
  </div>
    <% @product_images.each_with_index do |p| %>
      <!-- 顯示所有商品圖片 - other 尺寸小圖 -->
      <div class="col-md-2 col-sm-4 col-xs-6 image-box">
        <%= image_tag(p.image.thumb.url, class: 'list-image') %>
      </div>
    <% end %>
  <% else %>
    <%= image_tag('http://placehold.it/300x300&text=No Pic', class: 'thumbnail') %>
  <% end %>
</div>