Repositories

This is the explanation to what was happening:

My earlier work flow was "wrong" according to Moodle. This is what I was doing:
- upload the images to a Folder resource first
- whenever I need an image, visit the Folder, get the link to the image I need
- paste it to the <img> tag in HTML

That confuses Moodle. This is what it expects:
- the first time I need an image upload it in the GUI editor
- whenever I need it again, select it in the File picker (again GUI)

Painful for me (because I do not use the GUI editor). But that is how it is.