I use GIMP:
http://www.gimp.org/
"GIMP is an acronym for GNU Image Manipulation Program. It is a freely
distributed program for such tasks as photo retouching, image
composition and image authoring.
"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.
"GIMP is written and developed under X11 on UNIX platforms. But
basically the same code also runs on MS Windows and Mac OS X."
Bob