fog gemを使って、特定のURLのファイルをS3へ保存する ref: http://qiita.com/items/7f3c53a3198a1c7636ef

s3.rb

require 'rubygems'
require 'open-uri'

storage = Fog::Storage.new(
  provider: 'AWS',
  aws_access_key_id:ENV['AWS_S3_KEY_ID'],
  aws_secret_access_key:ENV['AWS_S3_SECRET_KEY'],
  region: ENV['AWS_REGION']
)
bucket = storage.directories.get(ENV['AWS_S3_BUCKET'])
bucket = storage.directories.create(key: ENV['AWS_S3_BUCKET']) unless bucket

path_to_save = 'users/images/hoge2.png'
image_url = 'http://a0.twimg.com/profile_images/2900491556/9d2bf873770958647f18b8e61af31f1a_normal.png'

file = bucket.files.create(key: path_to_save, public: true, body: open(image_url))
puts file.public_url