deaddruid → created an issue.
Thank you for pointing me in the right direction. I was getting frustrated, as I couldn't seem to insert images. The problem has turned out to be that my images are all SVG format, and Drupal does not support SVG in the core functionality. Having now enabled SVG, I am able to proceed as you describe.
I've also now read the forum guidelines. My apologies for breaking the rules - I'm used to working with people who prefer to see an example, rather than have a problem described in words.
Mark