http://gimp.org/

The GNU Image Manipulation Program (GIMP), is a graphic editing tool that provides a set of tools almost as complete as Adobe’s Photoshop (without the hefty price tag). I use this software to meet all of my web and graphic design needs, and it does so simply and effectively.  One of the nice things about this software is that regular releases bring new features at a constant rate, meaning that if it does not do what you want today, it may do it in six months time.