@ruud Friendica uses BBCode internally, although it does support Markdown as well (it is parsed as BBCode, so that's what you will see when editing your posts).
The syntax indeed is [img=link/to/image (it will be an internal link if you upload the image straight to your friendica instance)]alt-text[/img].
@Ruud This is what I learned in three years of Friendica. I hope it can be useful to you
(this also needs to be translated from Italian)
Add img alt text for Accessibility · Issue #12650 · friendica/friendica
GitHub[img=] and [/img]. Cool. Tested it and works.