Thank you. Mea culpa *again*. The good news is that my image now shows up on mouseover. But not without paying a price for it. These are the consequences:
1. The paragraph immediately after this Div now attaches itself to the Div paragraph and refuses to separate, even after I force wrap it in p tags.
2. The Div paragraph was previously all bold with tags at the start and end. After adding the new code, the closing tag is auto-deleted and only the text preceding the code is bold; all text after the new code is resolutely normal.
3. Initially, the popup image overlaid my right sidebar and the text therein as well as the Search box were visible on top of my popup image. I assume I could have fixed this by changing some z attributes but I instead moved the image so that it popped up over the left side of my page where it bossed the z fight.
4. After adding the code, my visual editor looked like the dog's dinner because my 500x500px popup image was permanently visible there.
5. At viewports smaller than 979px, my popup image was permanently visible without mouseover. I assume that has something to do with me forcing my 3-bar menu to take over from my navbar menu in viewports smaller than 979px (which is a modification I don't want to sacrifice).
My conclusion, sadly, is that this prized mouseover functionality comes at too high a price for a novice to cope with and I will reluctantly, for now, give up on it.
Thank you for showing me how it could work though and I may return to it when I know what I'm doing.