GIMP (GNU Image Manipulation Program) is an open source image editor. Whether you need simple color correction to a photo, automatically seamlessly tiling textures, or custom a powerful image generator for various noise or fractal patterns, Gimp has a myriad of tools. It is available for Windows, Linux, and MacGIMP Website
GIMP is for manipulating images, and photographs.It has a dizzying array of noise and pattern generators. Compared to Krita, GIMP is more geared towards the dual purpose of photo manipulation, and generating custom mathematical models with its scripting framework. While GIMP does have an adequate brush engine, Krita is usually the better tool for drawing images from scratch.
GIMP is intuitive and responsive. If you are coming from the world of less-powerful image editors, such as the freely bundled Microsoft Paint, GIMP is easy to understand. The tools all do what they are supposed to. The interface was originally inspired by Photoshop, but features a wide degree of customization. You can either keep your entire workspace in one window, or undock tool panels and move them about freely.