OS X Daily reports: [edited]
GIMP is a popular free image editor from the Linux world that is like a freeware version of Photoshop, with tools for photo retouching and image manipulation
It’s a great free PS alternative, but Mac users have long had to install X11 to get it to run. But not anymore! The newest version of Gimp for Mac OS X is bundled as a self-contained native app. That means no X11 installations, no Xcode, nothing but a simple dmg download.
Drag Gimp to the /Applications/ folder like any other Mac app to install it, then launch as usual. If you have GateKeeper enabled you’ll want to right-click Gimp and choose 'Open' to temporarily get around Gatekeeper‘s developer restrictions in OS X Mountain Lion.