Thanks @Akash – I now understand that all shortcodes on a page must be closed, and having implemented that change can confirm that it works as expected. I think this goes back to a time before Shortcoder was able to use enclosed content and therefore did not use closing tags, but my memory’s not that good so I’ll just say thanks for the helpful response. 😁
Incidentally, I’m sorry all of the images in my original post are broken. I used the image upload service that was linked from the New Topic page, and the separate IMG sources do work when used in isolation, but just not when embedded in a post.