gimp

GIMP (GNU Image Manipulation Program) is a freely distributed piece of software suitable for such tasks as photo retouching, image composition, and image authoring. It is a powerful piece of software with capabilities not found in any other free software product. It can be used as a simple paint program, an expert-quality photo-retouching program, an online batch-processing system, a mass production image renderer, or an image-format converter. GIMP is modular, expandable, and extensible. It is designed to be augmented with plug-ins and extensions to do just about anything. The advanced scripting interface allows everything from the simplest task to the most complex image-manipulation procedures to be easily scripted.