annotation.rb

module Paperclip
  class Annotation < Thumbnail

    attr_accessor :annotation_text

    def initialize(file, options = {}, attachment = nil)
      super

      @geometry             = options[:geometry]
      @annotation_text      = options[:annotation_text]
      @annotation_font_size = (options[:annotation_font_size] or 18)
      @annotation_position  = (options[:annotation_position] or 'SouthEast')
    end

    def transformation_command
      trans = []

      if not @annotation_text.nil? and not @annotation_text.empty?
        trans << "-gravity #{@annotation_position}"
        trans << "-pointsize #{@annotation_font_size}"
        trans << "-stroke '#000C' -strokewidth 2 -annotate 0 '#{@annotation_text}'"
        trans << "-stroke none -fill white -annotate 0 '#{@annotation_text}'"
      end

      super + trans
    end
  end
end

photo.rb

class Photo < ActiveRecord::Base
  attr_accessible :image
  belongs_to :post
  has_attached_file :image,
                    :processors => [:annotation],
                    :styles => {
                      :thumb => '160x120',
                      :medium => '300x300',
                      :large => {
                        :geometry => '700x700',
                        :annotation_text => 'annotation text',
                      },
                    }
end