GIMP

GIMP (the GNU Image Manipulation Program) is a superb free alternative to Adobe Photoshop, and comes with a huge array of professional-quality functions for fine-tuning images and photos, and creating your own artwork from scratch.
It includes layers, highly customizable brushes, filters and automatic image-enhancement tools, and support for a huge number of plugins (some pre-installed, and others available to download separately).Its active community of contributors means it’s in constant