From the Site:
Download GIMP (for Mac or Unix)"What is this GIMP?
The "GNU Image Manipulation Program" (originally the General Image Manipulation Program) sired by Spencer Kimball & Peter Mattis.
In their own words, "GIMP is our answer to the current lack of free (or at least reasonably priced) image manipulation software for Linux and UNIX in general."
It is a raster editor, which means that it performs operations directly on the pixels that make up the image, and not a vector editor. Other (proprietary) raster editors include Adobe Photoshop, Jasc Paintshop Pro and the humble Microsoft Paint."