nfortunately, Leopard's PHP installation does not ship with a GD module. If you use this PHP version, Drupal will report that it can't find a GD library (and no image processing will be performed either). There is a tutorial on the web that explains step by step how to compile GD2 for the stock PHP 5.2.4 that comes with Leopard; however, PHP segfaults after the installation (at least for me).

You know, for an operating system that is hailed by many as an alternative to Linux that just works, OS X isn't really all that great.