Some simple helpers to construct gravatar urls and <img/>-tags.

gravatar_helper.rb

module Gravatar
  def self.options; @options ||= {}; end
  
  module GravatarHelper
    def gravatar_url(mail, options = {})
      options = Gravatar.options.merge(options).stringify_keys!
      options.assert_valid_keys('size', 'rating', 'default')
      "#{build_gravatar_host(mail)}/avatar/#{Digest::MD5.hexdigest(mail.to_s.strip.downcase)}.jpg#{build_gravatar_url_options(options)}"
    end
    
    def gravatar_tag(mail, options = {})
      gravatar_opts = { :size => options[:size] ||= Gravatar.options[:size] || 80, :default => options.delete(:default), :rating => options.delete(:rating) }
      options[:size] = "#{options[:size]}x#{options[:size]}"
      options[:alt] ||= mail
      image_tag(gravatar_url(mail, gravatar_opts), options)
    end    
    
    private
      def build_gravatar_host(mail, secure = false)
        secure ? "https://secure.gravatar.com" : "http://#{%w{0 1 2 www}[mail.length % 4]}.gravatar.com"
      end
      
      def build_gravatar_url_options(options = {})
        url_opts = []
        url_opts << "s=#{CGI.escape(options['size'].to_s)}" if options['size'].to_i > 0
        url_opts << "r=#{CGI.escape(options['rating'].to_s.downcase)}" if options['rating']
        url_opts << "d=#{CGI.escape((options['default'].respond_to?(:call) ? options['default'].call(options) : options['default']).to_s)}" if options['default']
        "?" << url_opts.join('&') unless url_opts.empty?        
      end
  end
end