【Rails】Paperclipで保存した画像のサムネイルの取得方法 ref: http://qiita.com/jacoyutorius/items/6844e72ece23dcac8560

file0.rb

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.integer :name
      t.attachment :icon
    end
  end
end

file1.rb

class User < ActiveRecord::Base

[:icon].each do |col|
        has_attached_file col , :styles => {:medium => "300x300", :thumb => "100x100"}, 
            :default_url => "./images/:style/missing.png",
            :url => "/images/:class/:attachment/:id_partition/:style/:filename"
        validates_attachment_content_type col,
            :content_type => ["image/png", "image/jpg", "image/jpeg"]
    end
end

file2.txt

irb(main):003:0> User.first.icon(:thumb)
  User Load (0.4ms)  SELECT `users`.* FROM `users` ORDER BY `users`.`id` ASC LIMIT 1
=> "/images/users/icons/000/000/001/thumb/picture?1412045787"

sample.html.erb

<div>
    <%= image_tag(@user.icon(:thumb)) %>
</div>