GNU Image Manipulation Program (GIMP)

Main Website - https://www.gimp.org/

GIMP is a cross-platform image editor available for GNU/Linux, OS X, Windows and more operating systems. It is free software

Whether you are a graphic designer, photographer, illustrator, or scientist, GIMP provides you with sophisticated tools to get your job done. You can further enhance your productivity with GIMP thanks to many customization options and 3rd party plugins.

Download Link - https://www.gimp.org/downloads/  (We would recommend using the "Direct" link)

Tutorials - https://www.gimp.org/tutorials/

GIMP User Manual - https://docs.gimp.org/2.10/en/