The problem in uploading the image appears to have been specific to generating a thumbnail at LJ, and related to (possibly, not sure yet) a problem with copying the image onto a thumb drive for transfer between my two computers (one is not connected to the internet; one is.) The images looked OK when retrieved from the thumb drive full size, and even (in two cases) produced thumbnails when checked with a directory-thumbnail display. Two different thumb drives (same mfg, different sticks) produced images that appeared to be OK, and emailed OK to most people, but on close examination by a friend with more software than I have, showed a band of 17 pixels as "corrupt." When I switched to a different brand, the problem disappeared.
It would've been nice if LJ had been able to say "image corrupted enough to screw up thumbnail generation even though the full size looks good, but I'll be wary of these two thumb drives.