Image with padding for paperclip versions 2.3.1.1

padder.rb

module Paperclip
  class Padder < Thumbnail
    def transformation_command
      trans = []
      trans << " -gravity center"
      trans << " -background white"
      trans << " -extent " << %["#{geometry_extent}"]
      trans = trans.flatten.compact.join(" ")
      super << trans
    end

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

http://stackoverflow.com/questions/2728655/paperclip-resize-to-fit-a-rectangular-box