Just use preview. It's an awesome little app - included free with every copy of Leopard sold



! Use the 'select' button from the toolbar.
Select the area you want from your first photo, copy it, and then go into your second photo in Preview and past the selected parts and they'll appear in the new image. Remember to hold down the mouse button on the 'select' icon from the toolbar....then you can do all kinds of magic including the 'instant alpha' tool.
Cheers