Following Chase-san’s request, Woopsi’s BitmapIO class can now load 16-bit BMP files (in 555 or 565 pixel encoding, or indeed any other pixel encoding you can throw at it). It can also cope with V4 and V5 DIB headers in addition to V3.
I finally found a free graphics program for Windows that could save 16-bit images in the form of Pixelformer. I’ve used it for all of about 20 seconds but it looks like it might be a pretty decent pixel editor. It uses the V4 header format, unfortunately, hence the added support for V4 and V5 headers.
I haven’t had chance to do much testing yet, so there are probably bugs.