simple gravatar / avatar image model

avatar_image.rb

class AvatarImage
  GRAVATAR_IMAGE_BASE_URL     = 'http://www.gravatar.com/avatar/'
  GRAVATAR_IMAGE_DEFAULT_SIZE = '32'
  DEFAULT_URL                 = 'http://your-awesome-domain.com/images/your-awesome-default-image.png'

  attr_accessor :email

  def self.default_url
    DEFAULT_URL
  end

  def initialize(email)
    @email = email
  end

  # if no email has been set, returns the default avatar url
  # otherwise returns the generated gravatar_url
  # options include:
  #   size    => size of image to display, can be within the range "1" to "512"
  #              defaults to "32" if not specified.
  #              TODO: handle resizing of default avatar. as is, assumes the
  #              default size and / or passed option size will match default avatar
  #              imagess size. Could implement http://domain.com/images/avatar_{size}.png
  #   default => default image to display if no gravatar is found
  #              defaults to CW avatar man if not specified.
  def url(options = {})
    options[:default] ||= DEFAULT_URL
    options[:size]    ||= GRAVATAR_IMAGE_DEFAULT_SIZE
    if @email.blank?
      options[:default]
    else
      gravatar_url(options[:size], options[:default])
    end
  end

  private

  # return the gravatar image url for the specified email
  # specify image size and default url if no image is found
  def gravatar_url(size, default)
    "#{GRAVATAR_IMAGE_BASE_URL}#{gravatar_email_hash}?s=#{size}&d=#{CGI.escape(default)}"
  end

  # generate the gravatar image based on their specifications:
  #   1. Trim leading and trailing whitespace from an email address
  #   2. Force all characters to lower-case
  #   3. md5 hash the final string
  # via: http://en.gravatar.com/site/implement/hash/
  def gravatar_email_hash
    Digest::MD5.hexdigest(@email.strip.downcase).to_s
  end
end

example of usage in a view:
= image_tag AvatarImage.new(@comment.commenter_email).url