These functions should be defined by the gd module within PHP. You will need to ensure that you have the gd module enabled for php on your server. You can check the status of the gd library by creating a php info page. This is any php page with this line of code in it:
<?php phpinfo(); ?>
You should be able to see gd listed in the extensions along with the details about the version. Without this extension enabled you will get this error any time you reference these functions or others that are included in the library.