GIMP (the GNU Image Manipulation Program) is a mature, fully open-source raster editor and the best-known free alternative to Adobe Photoshop. It runs natively on Linux, Windows and macOS, and its feature set covers photo retouching, compositing, painting and format conversion.
As open-source software it is free forever with no subscription, and a large community of plug-ins and scripts extends it further. The interface differs from Photoshop and has a learning curve, but for a no-cost, privacy-friendly desktop editor it is hard to beat.
