gistfile1.rb

require 'rubygems'
require 'RMagick'

img = Magick::ImageList.new("test.jpg")
for y in 0..img.rows-1
  for x in 0..img.columns-1
    c = img.pixel_color(x, y)
    img.pixel_color(x,y, Magick::Pixel.new(c.red/2, c.green/2, c.blue/2))
  end 
end
img.write("test1.jpg")