That map wasn't 8 bit so I just changed it
If you use Paint you have to save the image to bmp 256 colours and then in PNG so it will be at 8 bit but you risk to lose a bit of quality, this is why I always save the maps in PNG and then i convert them with a software called "Easy graphic Converter".
If you use Photoshop, Gimp or similars you can easily convert your image at 8 bit