GIMP – (FREE) – https://www.gimp.org
GIMP is a cross-platform open-source image editor available for GNU/Linux, OS X, Windows and more operating systems. GIMP is an serious alternative to Photoshop in terms of both power and feature sets. Like Photoshop, there is a learning curve. I haven’t used it a whole bunch, so if you have questions about it, post on the forum and a GIMP user will be able to help you.