When I try to embed an image, a convenient way to do so is via the "landscape icon" in the edit UI. For at least some Wikipedia images, for example this one, I consistently get an error that looks like this:
Is this by design? If so, why?
After realizing that the original link I used was not an image, I tried to embed the actual image manually, using the Markdown content created by successful uploads, and it works:
This probably indicates that it is an Imgur issue.
Note that the image page on Wikipedia that opens when one clicks on an image in a Wikipedia article may not be an actual image; this was the URL that I used in the first attempt, which is not "embeddable" at all: https://en.wikipedia.org/wiki/Silicon_nitride#/media/File:Si3N4bearings.jpg. Despite the ".jpg" suffix the URL points to a relatively complex HTML web page, as an astute observer — like Glorfindel ;-) — would realize by the footer and navigation arrows.