For those who don't wish to spend a lot of money, Gimp comes free. You can use it for Mac, linux, windows. Gimp was first released in 1998. Two students at Berkeley, Spencer Kimball and Peter Mattis got the whole thing started. Neither had any experience with graphic's. Gimp stands for (Image Manipulation Program).
There are three builds for Gimp; so make sure which one will work with your Mac. It may require Apple's X11
Gimp has grown quite a bit from it's launch. Making it a quite powerful graphic's program. Gimp has layers, alpha channels, curves, a robust plug-in's. Gimp doesn't support CMYK or PANTONE color-spaces.
When your at the page; don't forget to click on the dog's face, this will bring you to the homepage for more info. Jon L