No image and wrong description while sharing a post

Hi.
When I share a post likie from here:
http://www.add-vantage.warszawa.pl/category/podstawowy-kurs-formy-24-tai-chi-stylu-yang/

There is no image from the embeded video.
And what’s more the description is inadequate.
It’s set stiff (it means it’s the same for sharing any post) and taken from I don’t know where, from my site.

I’ve set a correct description for all the posts on the page – each post has it’s own proper description. So why is it like that?

However the title of the sharing post is properly set – taken from the post title.
Then – I don’t know why – comes the site title – i’d like to get rid of it too.

Can you help me with this?