Just follow the steps:
Go to your PHP folder where your PHP files and extensions are located.
In most cases the location is C:PHP.
Open extensions folder and make sure that php_gd2.dll file is present in it.
If the file is there, skip to step 5.
You need to download the file mentioned in step 2 from http://php.net/
Copy the file in the extensions folder.
Open php.ini configuration file.
It's in C:PHP.
Choose either the php.ini – development or php.ini – production file, and rename it php.ini. (renaming is optional)
Enable php_gd2.dll extension from php.ini configuration file.
You can verify extension-dir through PHP info script.
If you don't have it already, just add the following code in a PHP file and save it as phpinfo.php.
Code for PHPinfo script:
Restart the IIS web server.