Add Open Graph Image Tags
Tags are explained here: https://ogp.me/
It should show provide some image when someone is sharing every page.
Previous versions had this tags included:
<meta property="og:image:type" content="image/png" />
<meta property="og:image:url" content="https://www.openalt.cz/2019/img/pozvanka-stehlik-1000.jpg"/><meta property="og:image:alt" content="Konference OpenAlt" />Most important are: