jekyll_image_fetcher.rb

# = Usage
# Download to your jekyll site directory, and run it
# `ruby jekyll_image_fetcher.rb`

Dir['_posts/*.md'].each do |path|
  content = File.open(path).read
  basename = File.basename path, '.md'

  content.gsub!(/!\[\]\((.+)\)/) do |m|
    url = $1
    `mkdir -p images/posts/#{basename}`
    `wget #{url} -P images/posts/#{basename}`
    m.gsub $1, "/images/posts/#{basename}/#{File.basename url}"
  end

  File.open(path, 'w') do |file|
    file.write content
  end
end