John,

I once had some B&W photographs I scanned and chose a number of shades of
gray greater than 256. Tried to load them onto a photo frame display and
they would not display but the color scans would. That tipped me to what
was going on.

I can't say what the difference exactly is. As mentioned, I've got some GIF
but also JPG images that are rejected by several programs but IE. Some of
them are B/W, others are color.

As for your situation, I cannot quite imagine what was causing it. A
limitation of the viewer to 256 color/grayshades perhaps ? But than why
didn't it replace some nearby grayshades to a single one ... Even my
thanwhile (DOS era) SEA image viewer did that, even for color images
(sometimes didn't look that great, but you could view them).

Regards,
Rudy Wieser