GIMP is a very powerful, free, open source image manipulation program. It stands
up to commercial programs costing hundreds, even thousand of dollars.
It can be used to create original art from a blank canvas. However, it is usually used to enhance, alter or combine Computer Generated Images and/or digital photographs.
I have found it invaluable. For example, I have rendered an image that is partially transparent, and then in GIMP superimposed that image on a background photograph, eliminating the more involved task of creating a background in the 3D software.
In another case, I had a number of lights in a complex 3D scene. Using LuxRender by itself it took 30 hours to get the quality I wanted, and then I had to change the scene. In the meantime, I watched a tutorial by Waldemar Belwon at Dreamlight suggesting that you render an image one light at a time, and combine the resulting images using a program like GIMP. I tried it, and 30 hours dropped to 3 hours!
I intend to list free and low-cost training resources for each software package. This will come eventually, but if I wait until everything is complete, I’ll never go live with this blog.
In the meantime, please include your favorite training resources in your comments.