OK, so it looks like to get full column width on-screen for a PC monitor (as opposed to a phone/tablet) you need to use the “Embed ” link with a larger image selected, and put up with the extraneous text, or upload directly from a file
Oh for heavens sake, the screenshot of the deleted text was there in the preview when I posted it, and also showed in the post after submission, even after re-loading the window.
edit, just for reference, this screenshot of the icon representing the missing screenshot, is still showing after re-loading the page.