GIMP is legendary. One of the first FOSS projects I ever used. I pirated Photoshop soon after trying it. Krita ( https://github.com/KDE/krita | https://krita.org/en/ ) is a more intuitive UX imo.
Krita is better in any conceivable way.
GIMP is the only image program I have seen on Linux that shows a preview when printing, and it lets the user freely scale the image to get the desired size on paper.