Most popular apps
GIMP is a free program for such photo retouching, image composition and image authoring. Essentially it's the closest thing to Photoshop you can get for free.
Inkscape is an Open Source vector graphics editor, with capabilities similar to Illustrator, CorelDraw, or Xara X, using the W3C standard Scalable Vector Graphics (SVG) file format.
Krita is a professional Free and open source (FOSS) painting program. It is made by artists that want to see affordable art tools for everyone withith concept art, texture and matte painters, illustrations and comics.
Blender is the free and open-source 3D creation suite. It supports the entirety of the 3D pipeline—modeling, rigging, animation, simulation, rendering, compositing, video editing, and motion tracking.
ShareX is a free and open source program that lets you capture or record any area of your screen and share it with a single press of a key. It also allows uploading images, text or other types of files to over 80 supported destinations.
Adobe Photoshop is the commercial image editor that set the standard in creative illustration and design work with a sophisticated, layer-based workflow and robust, professional feature set.
Paint.net is a free and very capable image editing software for Windows. Great alternative to Photoshop for people that do not need all the stuff PS offers.
Affinity Photo is a professional, full-featured raster graphics editor. Working in Affinity Photo is always live and you can pan and zoom at 60fps, with live previews and non destructive application.
Greenshot is a light-weight screenshot tool optimized for productivity with Optical Character Recognition (OCR) support.