An excerpt from the Rakefile I use to deploy my blog. http://blog.alexbrowne.info

aws_deploy_tools.rb

require 'rubygems'
require 'bundler/setup'
require 'openssl'
require 'digest/sha1'
require 'net/https'
require 'base64'
require 'aws-sdk'
require 'digest/md5'
require 'colored'


# A convenient wrapper around aws-sdk
# Also includes the ability to invalidate cloudfront files
# Used for deploying to s3/cloudfront
class AWSDeployTools
	
	def initialize(config = {})

		@access_key_id = config['access_key_id']
		@secret_access_key = config['secret_access_key']

		# for privacy, allow user to store aws credentials in a shell env variable
		@access_key_id ||= ENV['AWS_ACCESS_KEY_ID']
		@secret_access_key ||= ENV['AWS_SECRET_ACCESS_KEY']

		@bucket = config['bucket']
		@acl = config['acl']
		@cf_distribution_id = config['cf_distribution_id']
		@dirty_keys = Set.new # a set of keys that are dirty (have been pushed but not invalidated)

		if @bucket.nil?
    	raise "ERROR: Must provide bucket name in constructor. (e.g. :bucket => 'bucket_name')" 
	  end

	  if @cf_distribution_id.nil?
	    puts "WARNING: cf_distribution_id is nil. (you can include it with :cf_distribution_id => 'id')\n Skipping cf invalidations..."
	  end


		@s3 = AWS::S3.new(config)

	end

	# checks if a local file is in sync with the s3 bucket
	# file can either be a file object or a string with a
	# valid file path
	def synced?(s3_key, file)

		if file.is_a? String
			file = File.open(file, 'r')
		end

		f_content = file.read
		obj_etag = ""

		begin
	    obj = @s3.buckets[@bucket].objects[s3_key]
	    obj_etag = obj.etag # the etag is the md5 of the remote file
	  rescue
	   return false
	  end

		# the etag is surrounded by quotations. chomp removes them
	  obj_etag = obj_etag.gsub('"', '')

		# compare the etag to the md5 hash of the local file
	  obj_etag == md5(f_content)

	end

	# pushes (writes) the file to the s3 bucket at location
	# indicated by s3_key.
	# file can either be a file object or a string with a
	# valid file path
	# options are any options that can be passed to the
	# write method. 
	# See http://docs.aws.amazon.com/AWSRubySDK/latest/frames.html
	def push(s3_key, file, options = {})

		if file.is_a? String
			file = File.open(file, 'r')
		end

		# detect content type		
		require 'mime/types'
		file_path = file.path
		# remove the .gz extension so the base extension will
		# be used to determine content type. E.g. we want the
		# type of index.html.gz to be text/html
		if file_path.include? ".gz"
			file_path.gsub!(".gz", "")
		end
		content_type = MIME::Types.type_for(File.extname(file_path)).first.to_s
		content_type_hash = {:content_type => content_type}
		options.merge! content_type_hash
		
		puts "--> pushing #{file.path} to #{s3_key}...".green
    obj = @s3.buckets[@bucket].objects[s3_key]
    obj.write(file, options)

    @dirty_keys << s3_key
    # Special cases for index files.
    # for /index.html we should also invalidate /
    # for /archive/index.html we should also invalidate /archive/
    if (s3_key == "index.html")
			@dirty_keys << "/"
    elsif File.basename(s3_key) == "index.html"
			@dirty_keys << s3_key.chomp(s3_key.split("/").last)
    end
	  
	end

	# batch pushes (writes) the files to the s3 bucket at locations
	# indicated by s3_keys. (more than one file at a time)
	# files can either be a file object or a string with a
	# valid file path
	# options are any options that can be passed to the
	# write method.
	# See http://docs.aws.amazon.com/AWSRubySDK/latest/frames.html
	def batch_push(s3_keys = [], files = [], options = {})
		
		if (s3_keys.size != files.size) 
			raise "ERROR: There must be a 1-to-1 correspondence of keys to files!"
		end

		files.each_with_index do |file, i|

			s3_key = s3_keys[i]
			push(s3_key, file, options)
		  
		end
	end

	# for each file, first checks if the file is synced.
	# If not, it pushes (writes) the file.
	# options are any options that can be passed to the
	# write method.
	# See http://docs.aws.amazon.com/AWSRubySDK/latest/frames.html
	def sync(s3_keys = [], files = [], options = {})
		
		if (s3_keys.size != files.size) 
			raise "ERROR: There must be a 1-to-1 correspondence of keys to files!"
		end

		files.each_with_index do |file, i|

			s3_key = s3_keys[i]
	    unless synced?(s3_key, file)
				push(s3_key, file, options)
	    end
		  
		end

	end


	# a convenience method which simply returns the md5 hash of input
	def md5 (input)
	  Digest::MD5.hexdigest(input)
	end

	# invalidates files (accepts an array of keys or a single key)
	# based heavily on https://gist.github.com/601408
	def invalidate(s3_keys)

		if @cf_distribution_id.nil?
	    puts "WARNING: cf_distribution_id is nil. (you can include it with :cf_distribution_id => 'id')\n--> skipping cf invalidations..."
	    return
	  end

		if s3_keys.nil? || s3_keys.empty?
			puts "nothing to invalidate."
			return
		elsif s3_keys.is_a? String
			puts "--> invalidating #{s3_keys}...".yellow
			# special case for root
			if s3_keys == '/'
				paths = '<Path>/</Path>'
			else
				paths = '<Path>/' + s3_keys + '</Path>'
			end
		elsif s3_keys.length > 0
			puts "--> invalidating #{s3_keys.size} file(s)...".yellow
			paths = '<Path>/' + s3_keys.join('</Path><Path>/') + '</Path>'
			# special case for root
			if s3_keys.include?('/')
				paths.sub!('<Path>//</Path>', '<Path>/</Path>')
			end
		end

		# digest calculation based on http://blog.confabulus.com/2011/05/13/cloudfront-invalidation-from-ruby/
		date = Time.now.strftime("%a, %d %b %Y %H:%M:%S %Z")
		digest = Base64.encode64(OpenSSL::HMAC.digest(OpenSSL::Digest::Digest.new('sha1'), @secret_access_key, date)).strip

		uri = URI.parse('https://cloudfront.amazonaws.com/2010-08-01/distribution/' + @cf_distribution_id + '/invalidation')

		if paths != nil
		  req = Net::HTTP::Post.new(uri.path)
		else
		  req = Net::HTTP::Get.new(uri.path)
		end

		req.initialize_http_header({
		  'x-amz-date' => date,
		  'Content-Type' => 'text/xml',
		  'Authorization' => "AWS %s:%s" % [@access_key_id, digest]
		})
		 
		if paths != nil
		  req.body = "<InvalidationBatch>" + paths + "<CallerReference>ref_#{Time.now.utc.to_i}</CallerReference></InvalidationBatch>"
		end
		 
		http = Net::HTTP.new(uri.host, uri.port)
		http.use_ssl = true
		http.verify_mode = OpenSSL::SSL::VERIFY_NONE
		res = http.request(req)
		
		if res.code == '201'
			puts "Cloudfront Invalidation Success [201]. It may take a few minutes for the new files to propagate.".green
		else
			puts ("Cloudfront Invalidation Error: \n" + res.body).red
		end

		return res.code

	end

	# invalidates all the dirty keys and marks them as clean
	def invalidate_dirty_keys
		if @cf_distribution_id.nil?
	    puts "WARNING: cf_distribution_id is nil. (you can include it with :cf_distribution_id => 'id')\n--> skipping cf invalidations..."
	    return
	  end

		res_code = invalidate(@dirty_keys.to_a)
		# mark the keys as clean iff the invalidation request went through
		@dirty_keys.clear if res_code == '201'
	end


end

Rakefile

# ...

desc "Deploy website to s3/cloudfront via aws-sdk"
task :s3_cloudfront => [:generate, :minify, :gzip, :compress_images] do
  puts "=================================================="
  puts "      Deploying to Amazon S3 & CloudFront"
  puts "=================================================="

  # setup the aws_deploy_tools object
  config = YAML::load( File.open("_config.yml"))
  aws_deploy = AWSDeployTools.new(config)

  # get all files in the public directory
  all_files = Dir.glob("#{$public_dir}/**/*.*")

  # we want the gzipped version of the files, not the regular (non-gzipped) version
  # excluded files contains all the regular versions, which will not be deployed
  excluded_files = []
  $gzip_exts.collect do |ext|
    excluded_files += Dir.glob("#{$public_dir}/**/*.#{ext}")
  end

  # we do gzipped files seperately since they have different metadata (:content_encoding => gzip)
  puts "--> syncing gzipped files...".yellow
  gzipped_files = Dir.glob("#{$public_dir}/**/*.gz")
  gzipped_keys = gzipped_files.collect {|f| (f.split("#{$public_dir}/")[1]).sub(".gz", "")}

  aws_deploy.sync(gzipped_keys, gzipped_files,
          :reduced_redundancy => true,
          :cache_control => "max_age=86400", #24 hours
          :content_encoding => 'gzip',
          :acl => config['acl']
          )

  puts "--> syncing all other files...".yellow
  non_gzipped_files = all_files - gzipped_files - excluded_files
  non_gzipped_keys = non_gzipped_files.collect {|f| f.split("#{$public_dir}/")[1]}

  aws_deploy.sync(non_gzipped_keys, non_gzipped_files,
          :reduced_redundancy => true,
          :cache_control => "max_age=86400", #24 hours
          :acl => config['acl']
          )
  
  # invalidate all the files we just pushed
  aws_deploy.invalidate_dirty_keys  

  puts "DONE."

end

desc "Compress all applicable content in public/ using gzip"
task :gzip do

  unless which('gzip')
    puts "WARNING: gzip is not installed on your system. Skipping gzip..."
    return
  end

  @compressor ||= RedDragonfly.new

  $gzip_exts.each do |ext|
    puts "--> gzipping all #{ext}...".yellow
    files = Dir.glob("#{$gzip_dir}/**/*.#{ext}")
    files.each do |f|
      @compressor.gzip(f)
    end
  end

  puts "DONE."
end

desc "Minify all applicable files in public/ using jitify"
task :minify do
  
  unless which('jitify')
    puts "WARNING: jitify is not installed on your system. Skipping minification..."
    return
  end

  @compressor ||= RedDragonfly.new

  $minify_exts.each do |ext|
    puts "--> minifying all #{ext}...".yellow
    files = Dir.glob("#{$minify_dir}/**/*.#{ext}")
    files.each do |f|
      @compressor.minify(f)
    end
  end

  puts "DONE."

end

desc "Compress all images in public/ using ImageMagick"
task :compress_images do

  unless which('convert')
    puts "WARNING: ImageMagick is not installed on your system. Skipping image compression..."
    return
  end

  @compressor ||= RedDragonfly.new

  $compress_img_exts.each do |ext|
    puts "--> compressing all #{ext}...".yellow
    files = Dir.glob("#{$compress_img_dir}/**/*.#{ext}")
    files.each do |f|
      @compressor.compress_img(f)
    end
  end

  puts "DONE."

end

# ...

##
# invoke system which to check if a command is supported
# from http://stackoverflow.com/questions/2108727/which-in-ruby-checking-if-program-exists-in-path-from-ruby
# which('ruby') #=> /usr/bin/ruby
def which(cmd)
  system("which #{ cmd} > /dev/null 2>&1")
end

red_dragonfly.rb

# A set of tools for minifying and compressing content
# 
# Currently supports:
#   	- miniifying js, css, and html
# 		- gzipping any content
#     - compressing and shrinking images
#
# Currently depends on 3 system command line tools:
#			- ImageMagick (http://www.imagemagick.org/script/index.php)
# 		- gzip        (http://www.gzip.org/)
#     - jitify      (http://www.jitify.com/)
#
# These may be swapped out for gem versions in the future,
# but for now you have to manually install command line tools
# above on your system if you don't already have them.
#
# Author: Alex Browne

class RedDragonfly

	def initialize
		# ~~ gzip ~~
		$gzip_options = {
			:output_ext => "gz"
		}

		# ~~ minify (jitify) ~~
		# $minify_options = {
		#
		# }
		

		# ~~ images (ImageMagick) ~~
		# exts          : files extensions which sould be minified during batch operations
		# output_ext    : file extension of the output file ("" means keep the same extension)
		# max_width     : max width for compressed images
		# max_height    : max height for compressed images
		# quality       : image compression quality (1-100, higher is better quality/bigger files)
		# compress_type : type of compression to be used (http://www.imagemagick.org/script/command-line-options.php#compress)
		$img_options = {
			:output_ext => "jpg",
			:max_width => "600",
			:max_height => "1200",
			:quality => "65",
			:compress_type => "JPEG"
		}
	end

	# accepts a single file or an array of files
	# accepts a file object or the path to a file (a string)
	# perserves the original file
	# the output is (e.g.) .html.gz
	def gzip (files = [])

		unless which('gzip')
			puts "WARNING: gzip is not installed on your system. Skipping gzip..."
			return
		end

		unless files.is_a? Array
			files = [files]
		end

		files.each do |file|

			fname = get_filename(file)

		  # invoke system gzip
		  system("gzip -cn9 #{fname} > #{fname + '.' + $gzip_options[:output_ext]}")
		end

	end

	# accepts a single file or an array of files
	# accepts a file object or the path to a file
	# overwrites the original file with the minified version
	# html, css, and js supported only
	def minify (files = [])

		unless which('jitify')
			puts "WARNING: jitify is not installed on your system. Skipping minification..."
			return
		end

		unless files.is_a? Array
			files = [files]
		end

		files.each do |file|

			fname = get_filename(file)

		  # invoke system jitify
		  system("jitify --minify #{fname} > #{fname + '.min'}")
		  # remove the .min extension
		  system("mv #{fname + '.min'} #{fname}")

		end

	end

	# compresses an image file using the options
	# specified at the top
	# accepts either a single file or an array of files
	# accepts either a file object or a path to a file
	def compress_img (files = [])
		
		unless which('convert')
			puts "WARNING: ImageMagick is not installed on your system. Skipping image compression..."
			return
		end
		
		unless files.is_a? Array
			files = [files]
		end

		files.each do |file|

			fname = get_filename(file)

			compress_cmd = "convert #{fname} -resize #{$img_options[:max_width]}x#{$img_options[:max_height]}\\>" + 
		  					" -compress #{$img_options[:compress_type]} -quality #{$img_options[:quality]}" + 
		  					" #{get_raw_filename(fname) + '.' + $img_options[:output_ext]}"

		  # invoke system ImageMagick
		  system(compress_cmd)
		  # remove the old file (if applicable)
		  if (get_ext(fname) != ("." + $img_options[:output_ext]))
		  	system("rm #{fname}")
		  end

		end

	end

	# returns the filename (including path and ext) if the input is a file
	# if the input is a string, returns the same string
	def get_filename (file)
		if file.is_a? File
			file = file.path
		end
		return file
	end

	# returns the extension of a file
	# accepts either a file object or a string path
	def get_ext (file)

		if file.is_a? String
			return File.extname(file)
		elsif file.is_a? File
			return File.extname(file.path)
		end

	end

	# returns the raw filename (minus extension) of a file
	# accepts either a file object or a string path
	def get_raw_filename (file)
	
		# convert to string
		file = get_filename(file)
		# remove extension
		file.sub(get_ext(file), "")
		
	end

	##
	# invoke system which to check if a command is supported
	# from http://stackoverflow.com/questions/2108727/which-in-ruby-checking-if-program-exists-in-path-from-ruby
	# which('ruby') #=> /usr/bin/ruby
	def which(cmd)
	  system("which #{ cmd} > /dev/null 2>&1")
	end

end

I think there could be an issue with
def compress_img (files = [])
in that $img_options[:output_ext] is being forced to jpg by
$img_options = {
:output_ext => "jpg",
:max_width => "600",
:max_height => "1200",
:quality => "65",
:compress_type => "JPEG"
}

This means png files are being converted to jpg and deleted.