Trying to upload a picture in an answer to [this][1] question, I cannot upload images

![enter image description here][2]

but I can add links to images.

The answer looks fine (although not yet finished) - here is a screenshot of how I see it:

![enter image description here][3]

 but when posting I get an error like this:

![enter image description here][4]

What could be the cause of this?


  [1]: https://askubuntu.com/q/39849/47206
  [2]: https://i.sstatic.net/WmLfo.png
  [3]: https://i.sstatic.net/Ucb9M.png
  [4]: https://i.sstatic.net/6CuXe.png