I only thought of using alt-text today, and this works. While the image is still loading, my post can show a message about it.
Example
(press Shift-reload to see it in action)
My suggestion: is it possible to set this alt-text as a default for all embedded images?
Throbber test
Update for (manual) further improvements to embedded images: if you set the exact image size, the post "reserves" the required space even before the image is loaded.
Example:
(press Shift-reload to see it in action)