As you may know, GIMP (GNU Image Manipulation Program) is a free and open-source image editing software used a lot on Linux systems, as a free alternative of Adobe Photoshop (that can be used hand-in-hand with Inkscape).
It is widely used by photographers, graphic designers, illustrators and digital artists, providing a set of tools for tasks like photo retouching, image composition, and graphic design. It also includes layer masking, color corection and digital painting, making the tool very powerful.