For photo/graphic manipulation, Adobe Photoshop or Fireworks is great. However, as teachers, our use of these applications may not justify their expensive price tags. Gimp is an alternative for those of us who cannot afford the Adobe suite.
Gimp was developed under the GNU Project, a mass collaboration project started in the 80’s by Richard Stallman of MIT.
Gimp is entirely free to use.
Gimp has a learning curve, like any software; however, once you get used to its idiosyncrasies I believe you will find Gimp to be a powerful alternative to expensive image editing software.