This neat little program will allow you to get some very interesting effects on your images. Currently it supports Targa (TGA) images without compression, but it is pretty fun to use. Take the following image for example.
Below are some of the effects you can achieve by using this application, in conjunction with the image editing software of your choice.
In the first image, I set a high difference threshold so that only colors that were very different from each other would be saved. This creates a distinct, cell shaded appearance.
The second image is nothing special, and is to emphasize the third image. In the third image, I changed the palette to black and white, but kept all the green colors and then shifted them to a redder hue.
You can find the application and source code online at the following link. I will be adding support in the future for different image formats. Be sure to read the instructions so you know how to use the application.