watermark.rb

class User
  has_attached_file :photo,
                    :processors => [:watermark],
                    :styles => {
                      :medium => {
                        :geometry       => "300x300>",
                        :watermark_path => "#{Rails.root}/public/images/watermark.png"
                      },
                      :thumb => "100x100>",
                    }
end

# Rails.root /lib/paperclip_processors/watermark.rb
module Paperclip
  class Watermark < Thumbnail
    def initialize(file, options = {}, attachment = nil)
      super
      @watermark_path = options[:watermark_path]
      @position       = options[:position].nil? ? "SouthEast" : options[:position]
    end

    def make
      src = @file
      dst = Tempfile.new([@basename].compact.join("."))
      dst.binmode

      return super unless @watermark_path

      params = "-gravity #{@position} #{transformation_command.join(" ")} #{@watermark_path} :source :dest"

      begin
        success = Paperclip.run("composite", params, :source => "#{File.expand_path(src.path)}[0]", :dest => File.expand_path(dst.path))
      rescue PaperclipCommandLineError
        raise PaperclipError, "There was an error processing the watermark for #{@basename}" if @whiny
      end

      dst
    end
  end
end