Discourse probably sees the .png extension of the link and thinks ‘OK this is an image’. But in fact it’s not an image, it’s a MediaWiki page. So fails to embed a page as an image.
^ you can see above I paste
https://www.whonix.org/w/images/0/05/Tor_Browser_Internal_Updater_Popup.png rather than the MediaWiki URL
https://www.whonix.org/wiki/File:Tor_Browser_Internal_Updater_Popup.png and so it works.
Would just ignore. Not worth our while to try and fix in Discourse. Might report a bug upstream.