Oh, thank you so, so much, folks! I thought I'd clicked on every possible menu option -- and I had -- but it turns out I had downloaded the wrong version of Sigil (I had a way, way out-of-date version). I never would have realized it without knowing there was a menu item that didn't exist for me! (Although I should have realized when all my attempts to get to the help menu or manual were dead links...)
Anyway, I got the correct latest version, found the menu item for adding an image as you said, and have inserted my image. Thanks loads!
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