Thank you for testing the images on different devices. I had thought it was only a problem with Sony Reader for PC, but it looks like a much more widespread difficulty.
As I said in the first post, I never used 16-shade greyscale before, but I had seen several member post here that they use that for illustrations, and I experimented. Makes nice small png files, and they look great on computer screen in ADE and in my graphics editor, but they are not too useful if they aren't reliable on ereaders, and other software. So, yeah, back to 256-color for this project, or I will never get it uploaded!!!
Your thought to move this to the workshop is very canny. I will ask the mods to move me.
Thank you again for taking the time and trouble to investigate.