Check and install (enable) php GD support of your webserver
GD library allows you to write and use php scripts witch can manipulate images. It is used for such services as CAPTCHAS or image watermarking on-the-fly without modifying images itself.
At first make sure if your webserver (hosting or personal machine) supports GD library. Download this gd-test file. Extract and upload gd-test.php to your site’s root and access it through localhost (or http protocol).
If your browser shows image with word: works, then as you can guess your php installation has GD library enabled.
If this is not the case, then you have to manually enable GD library.
Open your php.ini file, which normally is in C:\WINDOWS directory and simply uncomment (remove “;”) the necessary line.
So line
;extension=php_gd2.dll
extension=php_gd2.dll
"captcha", "php", apache, GD library, http protocol, Microsoft Windows