responsive_image_helper.rb

module ResponsiveImageHelper
  def responsive_image_tag(srcset, **options)
    # Sort the images in the srcset by their value (Their pixel or '1x' width. Either should sort properly).
    sorted_srcset = srcset.sort_by { |_, value| value }

    # Use the smallest image in the given set as the image fallback. Might not be necessary with Picturefill.
    options[:src] = image_path_or_url_for_string(sorted_srcset.first[0])

    # Build the srcset attribute. Iterates through the given srcset hash, if a key contains '//' it
    # is assumed to contain a URL and will not use Rails image_path.
    options[:srcset] = sorted_srcset.map { |k, v| "#{image_path_or_url_for_string(k)} #{v}w" }.join(', ')

    # Creates a little DSL for the sizes option. The gte_sm, gte_md, etc keys line up with
    # Bootstraps default breakpoints. You could define your breakpoints somewhere else that
    # makes sense and adapt this to your needs.
    # 
    # Build the sizes attribute if necessary. If the sizes option is a string, just
    # pass that through to the final img tag.
    if options[:sizes].present? && options[:sizes].is_a?(Hash)
      options[:sizes] = options[:sizes].map do |media_query, length|
        if media_query.is_a? Symbol
          case media_query
          when :gte_sm
            "(min-width: 768px) #{length}"
          when :gte_md
            "(min-width: 992px) #{length}"
          when :gte_lg
            "(min-width: 1200px) #{length}"
          when :default
            length.to_s
          else
            nil
          end
        else
          "#{media_query} #{length}"
        end
      end.join(', ')
    end

    # Return the responsive image tag
    tag(:img, options)
  end

  # Determines if a given string contains a URL or a path by checking 
  # for the presence of //.
  def image_path_or_url_for_string(image_str)
    image_str =~ /\/\// ? image_str : image_path(image_str)
  end
end

Nice. It's worth pointing out that Rails image_tag supports srcset now: https://api.rubyonrails.org/classes/ActionView/Helpers/AssetTagHelper.html#method-i-image_tag although your helper does add some neat features.