I again tried to put some HTML in a NOTE on an OBJE record. This of course was altered so that the viewers see the tags. However, I could not find a way to edit it--I am only offered the chance to add another.
Therefore, I decided to delete it and replace it. The deletion did the right things in the GEDCOM, but I was not aware that it did NOT delete the actual image file.
When I click save on the replacement (same file, different NOTE), I got
Media file already exists. media/Patison-French.jpg
Blank name or illegal characters in name
Now, I understand the first line, though I think it shouldn't happen--just replace it (or maybe give a chance-to-back-out pop-up). But apparently, that somehow triggers the second line which clearly is not true.