Never mind - after looking at the slick theme, I realised that partials/breadcrumbs is being included at the top of the file, so this is the intended target 🙂
topic thumbnail doesn't show on facebook share
This is what the facebook debugger says.
The provided 'og:image' properties are not yet available because new images are processed asynchronously. To ensure shares of new URLs include an image, specify the dimensions using 'og:image:width' and 'og:image:height' tags. Learn More
Looks like we need to add those to our header.