Rich Text and Rich Embeds are very different extensions.
Rich Text is a free extension by askvortsov. It replaces the Flarum post editor with a WYSIWYG interface. I don't think it has been brought up in this discussion at all, and has nothing to do with it.
Rich Embeds is a paid extension created by myself that creates links previews and has a special feature to also augment image links with metadata as well as pre-fetch the width and height.
A WYSIWYG editor (but also the native editor) could also have an opportunity to address this issue pre-publication. Since the image is loaded in the preview of the post while it is written, the image link could be automatically converted into a bbcode image with hard-coded width and height before the post gets submitted. This value could be tempered with by the writer, but it wouldn't be dangerous as editing this value would just screw up the width/height ratio of your own images.