<%# no need for thumbnails unless there is more than one image %>
<% if @schwein.images.size > 1 %>
  <ul id="product-thumbnails" class="thumbnails inline" data-hook>
    <% @schwein.images.each do |i| %>
      <li class='tmb-all' id='tmb-<%= i.id %>'>
        <%= link_to(image_tag(i.attachment.url(:mini)), i.attachment.url(:product), :class => 'tmb-all', :id => "tmb-#{i.id}") %>
      </li>
    <% end %>
  </ul>
<% end %>

<%# no need for thumbnails unless there is more than one image %>
<% if @schwein.images.size > 1 %>
  <ul id="product-thumbnails" class="thumbnails inline" data-hook>
    <% @schwein.images.each do |i| %>
      <li class='tmb-all' id='tmb-<%= i.id %>'>
        <%= link_to(image_tag(i.attachment.url(:mini), size: '100x100'), i.attachment.url(:product), :class => 'tmb-all', :id => "tmb-#{i.id}") %>
      </li>
    <% end %>
  </ul>
<% end %>

Make the thumbnail size fix.
Make the thumbnail size fix.