gravatar.rb

# include the MD5 gem, this should be part of a standard ruby install
require 'digest/md5'

module Jekyll

  class Gravatar < Liquid::Tag

    def initialize(tag_name, size, token)
      super
      @size = size.strip
    end

    def render(context)
      # get the site config variables
      site_config = context.registers[:site].config

      # get the email address from the site config
      email_address = site_config['gravatar_email']
      # change the email address to all lowercase
      email_address = email_address.downcase

      # create an md5 hash from the email address
      gravatar_hash = Digest::MD5.hexdigest(email_address)

      # compile the full Gravatar URL
      image_src = "http://www.gravatar.com/avatar/#{gravatar_hash}"

      # append size query to URL if provided in tag
      unless @size.empty?
        image_src = image_src+"?s=#{@size}"
      end

      # output the full Gravatar URL
      image_src
    end

  end

end

# register the "gravatar_image" tag
Liquid::Template.register_tag('gravatar_image', Jekyll::Gravatar)