Hi all… I’ve worked out a solution…
When doing a new post:
Rename your image so it has at least a word or two of descriptive text in its title.
Upload it.
Click “Block” on the right panel.
A box showing “Alt Text” should show under “Image Settings”. Fill in as many words as you like, describing the image. “This is an image of etc…”
Publish when ready…
(The whole idea is it’s part of a system to aid the visually impaired, so if they reach that part of your page, they get a description of your image… if the field is left empty, they get no information).
This seems to work!