The GNU Image Manipulation Program
The GNU Image Manipulation Program (GIMP) is a powerful tool for the preparation and manipulation of digital images. The GIMP provides the user with a wide variety of image manipulation, painting, processing, and rendering tools. This is the GTK3 based development version leading to a future 3.0 release based on a recent snapshot of git master.
To install gimp3-devel, paste this in macOS terminal after installing MacPorts
sudo port install gimp3-devel