Untested but thought I would post it here just in case.
Will attempt to test it later on if I get time.

http://www.libgd.org/FAQ_PHP#How_do_I_get_gd_to_work_with_PHP.3F

Quote
Windows

The php_gd2.dll is included in a standard PHP installation for Windows, it's not enabled by default. You have to turn it on, the user may simply uncomment the line "extension=php_gd2.dll" in php.ini and restart the PHP extension.

Change:

#extension=php_gd2.dll

To:

extension=php_gd2.dll

You may also have to correct the extension directory setting from:

extension_dir = "./"

Or:

extension_dir = "./extensions"

NOTE: SUBSTITUTE THE ACTUAL PHP INSTALLATION DIRECTORY ON *YOUR* COMPUTER.

To (FOR WINDOWS):

extension_dir = "c:/php/extensions"

Thanks to Benoit Blais for this last point. Thanks also to Alan MacDougall and Perculator.