OK, <meta property="og:image" content="$wgLogo" /> is what I need to add to the header of each page, as my site has no images at all except for the logo. I use the latest git version of MediaWiki.
In OutputPage.php we see function addMeta( $name, $val ) { array_push( $this->mMetatags, array( $name, $val ) ); But then http://www.mediawiki.org/wiki/Manual:Hooks/BeforePageDisplay leads to http://www.mediawiki.org/wiki/ResourceLoader/Migration_guide_for_extension_d... which makes one utterly totally lost. OK, then I found http://www.mediawiki.org/wiki/Extension:OpenGraphMeta which of course does exactly what I want... and much more. But I don't to install any extensions. All I want is to add that one little line <meta property="og:image" content="$wgLogo" /> with the $ variable (expanded like $meta["og:image"] = wfExpandUrl($wgLogo); ) or something. So how do I write the one line hook please?!