The GIMP – Open Source Image Editing

The GIMP is a full featured, powerful and free image creation, photo retouching, painting, drawing and general image manipulation package.

It has many capabilities. 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, an image format converter, etc.

GIMP is 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.

It runs on most systems, in my case under Windows, which involves installing GTK first. All you need has been packaged up for download here.