How can I save a bmp (bitmap) image file at 16bpp? I need to save an image as Windows Bitmap BMP A1R5G5B5 16bpp.
Source files are 32bpp image files (PNG and/or Windows Bitmap).

Preview saves bmp images at 32 bits per pixel (bpp).

XnConvert saves bmp images at 24bpp.

Imagemagick, Acorn, GraphicConverter do not support saving images at 16bpp.

Photoshop can do it, but I don't own it.

Any ideas? 

GUI or command line is fine for me.
GIMP has the A1R5G5B5 option when using Export Image as BMP > Advanced Options



For images without any transparency A1R5G5B5 will be greyed out.
Use X1R5G5B5, also 16bpp format, for images without transparency.