Here is a
Paint-by-Numbers tutorial I found on the web.
In Short:
1. Open your image.
2. Reduce the colors by changing the mode to Indexed Colors or by Saving it for the web as a GIF File.
3. Make a duplicate of this image.
4. Filter>Stylize>Find Edges
5. Desaturate this Layer (CTRL+SHIFT+U)
5. Colorize the image with a blue tint using Hue/Saturation (CTRL+U)