This guide will help you to set your forum's open graph image. It will use the first image in your thread, whether linked directly or attached.
Open template: thread_view
Find:
Code:
<xf:set var="$fpSnippet" value="{{ snippet($firstPost.message, 0, {'stripBbCode': true}) }}" />
Add below:
Code:
<xf:if is="$thread.cover_image">
<xf:macro template="metadata_macros" name="metadata"
arg-description="{$fpSnippet}"
arg-shareUrl="{{ link('canonical:threads', $thread) }}"...