Thank you for the clues.
That link didn’t actually tell me a lot, but inspired me to kick off a search that found this document:
Qt Documentation : Qt 5.15 : Qt Core : C++ Classes : QStandardPaths
in which it says that the actual path (under Linux) is
PicturesLocation : "~/Pictures"
Other standard paths can be more complicated, e.g.:
FontsLocation : "~/.fonts", "~/.local/share/fonts", "/usr/local/share/fonts", "/usr/share/fonts"
AppLocalDataLocation : ~/.local/share/<APPNAME>, /usr/local/share/<APPNAME>, /usr/share/<APPNAME>
AppConfigLocation : ~/.config/<APPNAME>, /etc/xdg/<APPNAME>
What that document doesn’t say is that folders created in `~/Pictures’ are also searched for image files.
Curating images is a difficult problem as there are many different opinions on how best to do it, some of which are mutually exclusive. Much like many human endeavours.