Update to site software breaks something.
There was an update to the software used to run this website - Dokuwiki - and unbeknownst to me, fixed a fix I had made and broke something. To put that more clearly, I had made a fix to the template I use for layout etc., so it was friendly to Facebook. With the “Igor” update, the template was updated and my fix removed.
It was a real pain putting that fix back in the last couple of times, so I wasn't looking forward to it, but this time everything went a lot smoother than I thought.
Firstly, let me describe the problem. Whenever I shared a page on Facebook, the logo was displayed on the posting - the logo being my ugly black and white mugshot extrapolated from an x-face (an old technology for newsgroups and email). What Facebook does when you make a posting, is grab the raw page mark-up and hunt for the first image - and that's what it displays on the posting. Not very helpful when you've got a well chosen image on the page that is more pertinent to the article and it chooses to display the logo instead.
My solution was simple enough, embed an SVG version of the logo image in the page template - Facebook skips over these and finds the next image along, which is the image we want it to display.
With the Igor update, the template was also updated, so my code embedded in it was lost. Not entirely, I keep backups, I'm not entirely stupid.
However, the “Igor” update also allows SVG images to be used. All I had to do was rename my mugshot to “logo.svg” and upload it to the first place Dokuwiki looks for a logo. I also added a backup logo with a slightly different name, so should my personal logo get overwritten in the future (unlikely), I can simply copy the backup. Thank you Andreas, for making things a lot easier for me.