padder.rb

module Paperclip
  class Padder < Thumbnail
    def transformation_command
      super + ["-gravity center",
               "-background white",
               "-extent", %["#{geometry_extent}"]]
    end

    def geometry_extent
      "#{target_geometry.width.to_i}x#{target_geometry.height.to_i}"
    end
  end
end

user.rb

class User < ActiveRecord::Base
  has_attached_file :image,
    :styles => {
      :small => "50x50",
      :medium => "130x130>",
      :large => "280x280>",
      :haystacks => "330x330>" },
    :processors => [:padder]
end