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. Free • 189 alternatives • 3865 likes
Inkscape is a professional vector graphics editor for Linux, Windows and macOS. It's free and open source. Free • 128 alternatives • 1868 likes
Professional, free, and open-source raster graphics editor with advanced digital painting and sketching tools and workflow features. Free • 130 alternatives • 906 likes