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.
Result:

You could also manually type in the Markdown for an image, which looks something like...
.
Or other formats. If this is your thing, see additional information on Markdown editing at SE.