Hmm… interesting. The only time I’ve ever gotten that error is when I accidentally cut and pasted an image instead of an image URL. Meaning, I right clicked on the image I wanted to post, clicked on “Copy Image” instead of “Copy Image URL” and when I pasted it, text would show up with an image tag and a file name, but the preview would have the error mentioned in the OP.
Otherwise, I’ve never seen that error message. A plain URL always previews and posts fine for me. It’s when I’m in a hurry and copy the wrong thing that I see that.