@jac Good question. I raised something similar on the NodeBB community
https://community.nodebb.org/topic/15650/default-image-when-sharing-topics-posts
but, I never got a response - however, there is an OG image (Open Graph) which you can apply at
/admin/settings/general
Which (I think) should give you a default image (which you of course specify by uploading) for sharing on social media.
EDIT - on checking, this does indeed work. Once the image is applied, the Open Graph tags are present in each post - see below
<meta property="og:title" content="Social icon (Nodebb)" />
<meta property="og:description" content="@jac Good question. I raised something similar on the NodeBB community May 11 / Technical Support ..." />
<meta property="og:type" content="article" />
<meta property="article:published_time" content="2021-12-04T16:58:18.479Z" />
<meta property="article:modified_time" content="2021-12-04T17:28:05.608Z" />
<meta property="article:section" content="Customisation" />
<meta property="og:image" content="https://sudonix.com/assets/uploads/profile/1-profileavatar-1629822570834.png" />
<meta property="og:image:url" content="https://sudonix.com/assets/uploads/profile/1-profileavatar-1629822570834.png" />
<meta property="og:image" content="https://sudonix.com/assets/uploads/system/og-image.png" />
<meta property="og:image:url" content="https://sudonix.com/assets/uploads/system/og-image.png" />
<meta property="og:image:width" content="1900" />
<meta property="og:image:height" content="950" />
<meta property="og:url" content="https://sudonix.com/topic/162/social-icon-nodebb/2" />