@Jellby --- upon reading your post again, I realized that I didn't quite get your meaning the first time through. I didn't mean to sound argumentative, sorry.
@Tex2002ans --- Just for kicks and grins, here is another test-bed epub with identical images. They are 500px wide x 740 high (30% of the source image).
It includes 256-Greyscale (8-bit), 256 indexed palette (8-bit), 16 indexed (4-bit), 8 indexed (3-bit), 4 indexed (2-bit), and 2-color black & white (1-bit), AND, 16-color increased back to 256. (I threw in a couple of jpegs too, just for your viewing pleasure
)
Once again, ADE for desktop shows everything just fine, but SonyReader for desktop barfs on anything less than 8-bit. ---- Well, not quite, the Sony software display of the 2-color black-and-white image is good. Go figure...