gistfile1.rb

module ApplicationHelper
  include ActionView::Helpers::AssetTagHelper

  def image_tag(source, options = {})
    if options[:retina]
      options['data-at2x'] = path_to_image(source.split('.').join('@2x.'))
      options.tap { |h| h.delete(:retina) }
    else
      options['data-no-retina'] = ''
    end

    super(source, options)
  end

end