From Two Pilots:
Layer Pilot is color quantization software allowing you to apply the posterization image effect to your photos. To create an image poster using this program you should break up your image into separate color layers. Selected color layers can be merged to form a new version of your original image. This new image poster will consist of only a few colors of your choice! Every color layer can be saved as a separate image or can be exported as a layered PSD file. With Layer Pilot it is easy to change picture colors by adjusting a color for each color layer. The program supports the following graphic formats: BMP, JPEG, PNG.