Image preview (Featured Image) when sharing a link


Been getting to grips with Grav and so far so good.
I have the Quark theme installed.
When I share a link to my website on WhatsApp (for example), it shows a preview of the site details which I can see is information put into a couple of sections of the admin panel.
However the image shown is the Grav logo. I am struggling to find where this logo is coming from.
In WordPress, I would set a featured image on a page and that image would show up in previews when sharing a link to the page.
Any help appreciated.

Kind regards,