Discourse + Ghost integration and SEO

Hello, last question on this subject !
For the moment I have trouble displaying the pictures when I click on “see the full message” on the forum …
Probably because these are SVG images. But it still doesn’t work with PNG images.
I guess I just need to change the settings in “External integration settings”.
I don’t really understand the difference between “CSS selector for elements that will be allowed in embedded content” and “CSS classes allowed” … (translation problem maybe?).

I would like to understand what is catched for the post on the forum and what is displayed when we click on “see the full message”.

I do not find anything on this subject in the Discourse documentation.
Any idea on this?
Thank you !