Custom bit depth ver. 2.10.20

[rich2005]

I am not sure if it is possible with Gimp. You could try putting the image into indexed mode with a 16 colour pallete, then export as a .bmp (with compatibility mode off - maybe)

   

Otherwise you probably need to use ImageMagick see: http://www.imagemagick.org/discourse-ser...hp?t=35655

Best place to ask questions is now: https://github.com/ImageMagick/ImageMagick/discussions