GIMP
Open Source
GIMP stands for GNU Image Manipulation Program which is an opensource image editor program supporting almost every popular image formats.
RawTherapee
Open Source
RawTherapee is a cross-platform raw image processing program, released under the GNU General Public License Version 3.
photivo
Open Source
Photivo is a free and open source (GPL3) photo processor. It handles your RAW files as well as your bitmap files.
Photopea
Free
Photopea is a free online image editor, that runs without plugins. It can both read and save PSD files.
MARI
Commercial
MARI is a digital 3D painting and texturing platform.
Krita
Open Source
Krita is a professional free and open source painting program.
Polarr
Freemium
Polarr is a photo editing software.
ImageJ
Open Source
ImageJ is a public domain, Java-based image processing program.
Gimp Paint Studio
Open Source
GIMP Paint Studio is a great pack of designing components like brushes, dynamics, gradient etc.
Compare Fotor with alternatives
Compare Gimp Paint Studio and Fotor and decide which is most suitable for you.