Block-editor: Klik on the add icon (+). Click on the icon with the landscape. Upload an image, etc.
Thanks. I missed that completely. I assumed incorrectly that the “+” only added a paragraph, and that the three dots on the right-hand margin were the only formatting controls.
When the site is not 100% responsive you probably added some HTML or CSS code that conflicts with HTML or CSS code of the theme. Use the Developer Tools of the browser (F12) to find out what the problem is.
I’ll give the Developer Tools a try now. I have added quite a bit of CSS.. 🙄 😆