gem 'paperclip-compression'

class User < ActiveRecord::Base
  has_attached_file :avatar,
    styles: { medium: '300x300>', thumb: '100x100>' },
    processors: [:thumbnail, :compression]
end

class User < ActiveRecord::Base
  has_attached_file :avatar,
    styles: {
      medium: '300x300>',
      thumb: {
        geometry: '100x100>',
        processor_options: {
          compression: {
            png: false,
            jpeg: '-copy none -optimize'
          }
        }
      }
    },
    processors: [:thumbnail, :compression]
end

module YourApp
  class Application < Rails::Application
    # Other code...

    config.paperclip_defaults = { :compression => { :png => false, :jpeg => '-optimize' } }
  end
end

Paperclip::Attachment.default_options[:compression] = { :png => false, :jpeg => '-optimize' }