<% @campaign.weeks.each do |week| %>
            <% week.creatives.each do |creative| %>
                <%= image_tag(creative.image_url(:thumb), class: "img-thumbnail") if creative.image? %>
        <% end %>
   <% end %>

class Campaign < ActiveRecord::Base
   has_many :weeks
   has_many :creatives, through: :weeks
end

<% @campaign.creatives.each do |creative| %>
   <%= image_tag(...) %>
<% end %>