app-controllers-images_controller.rb

class ImagesController < ApplicationController
  skip_before_filter :ensure_logged_in

  def show
    id    = "%s.%s" % [params[:id], params[:format]]
    image = AdImage.find(id).decorate
    send_data image.contents, :type => image.mimetype, :disposition => "inline"
  rescue
    render :text => ''
  end

end

app-decorators-ad_image_decorator.rb

class AdImageDecorator < ApplicationDecorator
  delegate_all

  def mimetype
    "image/%s" % file_suffix
  end

  private

  def file_suffix
    id.sub(/.*\./, '')
  end
end