GIMP is a well-known photo manipulation tool that has been ported to the Mac operating system environment. Gimp for Mac is a free app that installs easily. Gimp for Mac is a relatively unique tool. It can be used as a paint program, or as a photo retouching tool, or as an image composition and manipulation program. It is hard to categorize Gimp for Mac except to say you'll probably want it if you do a lot of work with images.
The Gimp for Mac interface opens with a folder and file directory tree in the left pane, a set of thumbnail images in the middle pane, and a set of command icons and options across the top, right side, and bottom of the window. Once you select an image it is blown to full size and you can then use the comprehensive set of manipulation and touchup tools to alter the image and resave it. Learning the full power of Gimp for Mac will take months, but basic manipulation can be done in a few minutes once you figure out where the tools you need are.
Gimp for Mac works well. We've used it on other platforms for years, and having it on the Mac is just adding to the tool's utility. After years of development work, Gimp for Mac is a complete image manipulation tool and you'll find it as powerful and flexible as any other image tool on the market, but Gimp for Mac is free.