carrierwave.rb

# put this in config/initializers/carrierwave.rb
module CarrierWave
  module MiniMagick
    def quality(percentage)
      manipulate! do |img|
        img.quality(percentage)
        img = yield(img) if block_given?
        img
      end
    end
  end
end

image_uploader.rb

class ImageUploader < CarrierWave::Uploader::Base
  include CarrierWave::MiniMagick

  process :quality => 85

  version :medium do
    process :quality => 85
  end
end