Here is how to force your install to use the GD image processing library.

Open up the config/user_setup.php file and find this line:

// define('FORCE_GD', false);

Change that to:

define('FORCE_GD', true);

Be sure to remove the two forward slashes. Save the file and go to your preferences pane in Director. The "About your install" section should now report you are using GD1 or GD2, depending on your setup.