Currently Being ModeratedJul 7, 2013 11:29 PM (in response to scott oaks)
Editing capability comes from the app you use not the machine you run it on. Some examples of an image editor on the Mac:
The Gimp (free)
Graphic Coverter ($45 approx)
Acorn ($50 approx)
Pixelmator ($50 approx)
Photoshop Elements ($75 approx)
There are many, many other options. Search on MacUpdate or the App Store.
You can set Photoshop (or any image editor) as an external editor in iPhoto. (Preferences -> General -> Edit Photo: Choose from the Drop Down Menu.) This way, when you double click a pic to edit in iPhoto it will open automatically in Photoshop or your Image Editor, and when you save it it's sent back to iPhoto automatically. This is the only way that edits made in another application will be displayed in iPhoto.