I think this is indeed "operator error" :) But no big deal! It's fun to learn new features of how to make questions and answers even more helpful and pleasant to read.
See a Meta.SE question on adding images. The interface has been changed/improved since then.
A number of different things could be at work... e.g.,
- Your browser (Chrome, IE, Firefox, Safari, ...)
- Your browser settings (dis/allow scripts, pop-ups, security settings)
- Your OS or interface (Linux, OS X, Windows, mobile browser, Stack Exchange app)
- Simply getting used to the interface. :)
Here's an example: link to https://upload.wikimedia.org/wikipedia/en/thumb/8/80/Wikipedia-logo-v2.svg/103px-Wikipedia-logo-v2.svg.png as an image: click the "Add Image" button, then click "link from the web" link, then paste the URL, then click the "Add Picture" button.
You could also manually type in the Markdown for an image, which looks something like...
![alternate mouseover text here](https://i.stack.imgur.com/mg7TF.png).
Or other formats. If this is your thing, see additional information on Markdown editing at SE.