montage.rb

#!/usr/bin/env ruby

require "RMagick"

list = []

10.times do |i|
  list << "lena_#{i}.png"
end

list.each_slice(5).inject(Magick::ImageList.new) do |image_list, row|
  image_list << Magick::ImageList.new(*row).append(false)
  image_list
end.append(true).write("lena_tile.png")