I can't see your image. I think it's broken, but realistically you'll need to learn to do this yourself, anyway, right?
What image editor are you using? You can do it in Gimp or IRfan View, and probably others. In Gimp go to image > mode > indexed, then type in 16 where it says maximum number of colors..
In Irfan View go to Image > Decrease color depth, then select 16 colours and make sure Floyd-Steinberg Dithering is NOT selected.