Any 32bit operating system capable of running PHP5
PHP5, requires version >= 5.1.0 (it might work with earlier versions but this is not officially supported)
Enabled GD 2.x library in PHP installation. This is normally bundled with the PHP5 distribution. It is strongly recommended that you use the included version of GD with PHP5 and not try to install it separately.
There are known issues so running the library under a 64 bit OS so the library does not officially support any 64 bit OS:es
For more detailed information on PHP settings in php.ini see Verifying the PHP/GD installation