Correct.

or in steemit.com posts that will be shared to other social media platforms...is that right?

Well shared posts from steemit on other social media would just be a link to steemit and I believe it would target the cached image for a preview but I'm not sure on that. I guess you could share this post on Facebook and see what happens...

EDIT - Steemit.chat does not seem to use the cached images (it should however) Ill make a few calls ;)