The is an example to use ruby control command line tool(ImageMagick) to deal with image processing.

image_converter.rb

# encoding: UTF-8
#
# This program is used to convert all the png files under the given path, 
# from colorful to grey, from big to small(70% as original), from png to 
# gif, and then keep the file name unchanged(still *.png).
# 
# Running this program needs you have "imagemagick" installed.
#

require 'find'

unless ARGV.length == 1
  puts "Usage: image_converter.rb /home/user/images"
  exit
end

Find.find(ARGV[0]) do |f|
  if f.end_with?('.png')
    bw_name = f + '.bw.png'
    bw_gif_name = bw_name + '.gif'
    `convert -colorspace GRAY '#{f}' '#{bw_name}'`
    `rm '#{f}'`
    `convert '#{bw_name}' -resize 70% '#{bw_gif_name}'`
    `rm '#{bw_name}'`
    `mv '#{bw_gif_name}' '#{f}'`
    puts "convert '#{f}' ok."
  end
end