The difference seems to be not only in the number of shades of grey, but also in the
dithering. In FBreader it seems to be different and very strong. Look at the vertical/horizontal patterns in the fbreader image.
But there is a thing I don't understand :
The default image viewer is not implemented in the main reading application. So what is the use of this image viewer ? I don't think that many people bought a Pocketbook device to watch their family pictures in black and white. No ?
Anyway, thank you for your answer.