Have you looked at iPhoto? I can't remember if iPhoto does transparency.
But the Gimp is available for mac and easy to install. You can use Mac Ports or download it at the Gimp on Mac.
Gimp requires x11. Which is included with Snow Leopard. It's isn't a big deal to use or install. You do not have to do anything special to use it. Just launch the Gimp and it will launch x11.
If xll isn't on the machine, you can find it on the Snow Leopard install disc.
Let me stress this, you are not gonna find anything else that's open source (free) that does what the Gimp does. Running X11 is a SMALL price to pay to get all that free functionally.
There are several cheap alternatives to Photoshop too. Like Pixelmator. If you want a native mac app.