  ==========================================================
                        CAPTCHA
  ==========================================================

  Since version 1.7.3, 4images is able to protect forms by
  CAPTCHA.
  You'll find general informations about CAPTCHAS at:
  http://en.wikipedia.org/wiki/Captcha

  The following form can be protected by CAPTCHA:
    - Registration
    - E-Cards
    - Comments
    - Image-Upload

  The folder "captcha" contains all files for generating
  the CAPTCHA images.

  The folder "captcha/backgrounds" constain images
  for the background of the CAPTCHA images.
  A background is selected randomly from this folder.
  New images can be added to this folder and will be used
  for generation.

  The folder "images/fonts" contains fonts for generating
  the display CAPTCHA text.
  A font is selected randomly from this folder.
  New fonts can be added to this folder and will be used
  for generation.

  The file "captcha/wordlist.txt" contains a list of
  words which can be used instead of randomly generated
  strings. This function can be activated with the
  configuration option $captcha_wordfile.

  ----------------------------------------------------------
  How to enable the CAPTCHA system:
  ----------------------------------------------------------

  The CAPTCHA System is enabled by default.

  -- Advanced control of the CAPTCHA system ----------------

  You can control the CAPTCHA system in your config.php with
  the following configuration variables:

  - $captcha_enable = 1;
      A value of 1 enables the CAPTCHA system, 0 disables it.
      Default value is 1.

  - $captcha_enable_comments = 1;
      A value of 1 enables the CAPTCHA system for the comments
      form, 0 disables it.
      Default value is 1.

  - $captcha_enable_upload = 1;
      A value of 1 enables the CAPTCHA system for the upload
      form, 0 disables it.
      Default value is 1.

  - $captcha_enable_registration = 1;
      A value of 1 enables the CAPTCHA system for the
      registration form, 0 disables it.
      Default value is 1.

  - $captcha_enable_postcards = 1;
      A value of 1 enables the CAPTCHA system for the e-cards
      form, 0 disables it.
      Default value is 1.

  - $captcha_path = './captcha';
      Path to the folder which conatins the CAPTCHA files
      (backgrounds, fonts).

  - $captcha_chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
      A list of letters which will be used for the
      randomly generated CAPTCHA strings
      Default value is "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ".

  - $captcha_length = 5;
      Number of letters of the CAPTCHA strings
      Default value is 5.

  - $captcha_wordfile = 0;
      If this option is set to 1, a word from
      "captcha/wordlist.txt" is used.
      Default value is 0.

  - $captcha_width = 200;
      Width of the CAPTCHA image in pixel.
      Default value is 200.

  - $captcha_height = 70;
      Height of the CAPTCHA image in pixel.
      Default value is 70.

  - $captcha_text_color = "#000000";
      Color (Hex) of the CAPTCHA text.
      Default value is "#000000" (black).

  - $captcha_text_size = 25;
      Size of the CAPTCHA text.
      Default value is 25.

  - $captcha_text_transparency = 60;
      Transparency of the CAPTCHA text.
      Default value is 60.

  - $captcha_filter_text = 1;
      If this option is set to 1, the CAPTCHA text will
      be filtered ("obscured").
      Default value is 1.

  - $captcha_filter_bg = 1;
      If this option is set to 1, the CAPTCHA background
      image will be filtered ("obscured").
      Default value is 1.