digiKam - Professional Photo Management with the Power of Open Source
GIMP - GIMP is a multiplatform photo manipulation tool.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
Photos on macOS - Photos keeps your images organized and accessible. Rediscover favorite shots with Memories. Access them on your devices with iCloud Photos.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.