gistfile1.rb

# En el model.rb, nota que cambie created_at DESC por ASC
has_many :image_models, :as => :owner, :dependent => :destroy, :order => 'created_at ASC'

# agrega este metodo en le modelo model.rb
def first_image_permalink
  image = image_models.first
  image.permalink if image && !image.document_file_name.blank?
end

#Finalmente de la vista se llamaria asi
<% @models.each do |model| %>
   <%= image_tag model.first_image_permalink %>
<% end %>