Problem with posting images in the "Caption This" thread

Here’s a prior thread on this topic posted in the correct forum.

There was never a definitive answer, but I’ve linked to the first post that developed a reasonable theory of what’s probably the problem for most users when it happens. The punchline being confusion between trying to paste an image into your post (that will fail) versus trying to paste the url of an image into your post (that will / should succeed).