[derangedhermit]

256 shades are sufficient, but 16 is lacking (for displays that can display more shades and have sufficient contrast to make it worthwhile). Within 256 shades, there is probably enough room for useful transparency for these images.

For LCD screens, with sharply delineated pixels, some smoothing/ reduced sharpening on downsizing is needed to reduce aliasing. The need and amount can be seen and demonstrated using the "Color Quantizer" program, among others. The CQ program, for example, does resizing using one of many algorithms, with sharpness sliders.

E-ink screens need all the contrast you can give them, and, due to the softer pixel edges, can stand more sharpening than LCDs.

There are fundamental limits on the contrast that a display can show. Horizontal and vertical lines are one thing, diagonal another. Any diagonal line needs about 3 pixels average thickness or more to do fairly acceptable edges without artifacts. These specific images have many areas where the original image dimensions (e.g. 1800p wide) give good results, but downsizing to 1200 pixels (2 pixels/line) requires care and compromise, and 600 pixels gives an inevitable loss in image quality on any screen (in those areas). In some of the image areas, features are large enough to do better, though. For example, 021 has much area that tolerates shrinking, but 238 has much less.

I believe I now understand that the benefit of the png optimization programs is that one can work, largely without concern, in PNG24A, and then use one of these to produce an output image optimized in size and format (e.g. PNG8 or 8A without unneeded chunks) and close-to-optimally compressed. Many programs, including PSP and PS, do not support the full range of png options, or output optimal pngs, although many browsers and applications display them properly. Tools seem to support PNG8 and 8A (palettized /index color images) more widely than 8-bit grayscale.

I think these, in large, hi-res formats, would benefit from transparency when embedded in a book. I have no experience with transparency, but something like 50% for the white pixels, down to 0% for black, and scaled accordingly according to pixel values in between. I'd give up half the palette shades to implement transparency.