Setup for paperclip (https://github.com/thoughtbot/paperclip)

amazon_s3.yml

# config/amazon_s3.yml

development:
  access_key_id: <access key>
  secret_access_key: <secret>
test:
  access_key_id: ...
  secret_access_key: ...
production:
  access_key_id: ...
  secret_access_key: ...

YourModel.rb

# app/models/<Your Model>.rb
has_attached_file :<the_image_attribute_name>, :styles => { :medium => "300x300>", :thumb => "100x100>" }, :storage => :s3, :path => ":attachment/:id/:style.:extension", :bucket => '<your_image_bucket>', :s3_credentials => "#{::Rails.root.to_s}/config/amazon_s3.yml"