Global.php
cambiar las siguientes lineas
// Initialize CAPTCHA configuration
$captcha_enable = 1;
$captcha_enable_comments = 1;
$captcha_enable_upload = 1;
$captcha_enable_registration = 1;
$captcha_enable_postcards = 1;
$captcha_ttf = 1;
$captcha_path = ROOT_PATH.'captcha';
$captcha_chars = "2345689"; // aca elige que quieres que se vea el captcha letras o numeros
$captcha_length = 5;
$captcha_wordfile = 0; // cabia a valor 1 si quieres que cargue los textos de words.txt
$captcha_width = 200;
$captcha_height = 70;
$captcha_text_color = '#000000';
$captcha_text_size = 25;
$captcha_text_transparency = 60;
$captcha_filter_text = 1;
$captcha_filter_bg = 1;