Hello,
when we share a url, an image is retrieved from this url and shown in the post.
What is this image ? How can I set my website to show the image I want when someone share an url ?
Thanks for your help !
Have a good day !
The image, as well as the title, description text and other metadata, is fetched from the website itself using Open Graph metadata. If you want your website to be presented that way, look at the linked site and add those tags to your site! 
Tks a lot for your help Dennis !