It apparently has to do with the size of the image. These were both very large images, and I reduced the JPG quality to get the file size under 2 megabytes, which appears to be a limit that this forum imposes. The forum accepted the files, but does not display them, except as links to separately display the images.
I created new versions of these images, at smaller resolution, but my usual 95% JPG quality, and those upload and display just fine. I left the links in to the higher-resolution, lower-quality images, in case anyone wants to see those.
