Mr Papa said:
worked for me as guest…
I got excited for a second. 
But sadly, this is not what I'm looking for. 
You inserted an image, but you didn't upload it. It is a link to an image on another website. The thing that I'm trying to get to work is to upload the image to the forum and then insert it. So that the image will actually be hosted by the forum.
Simple:Press can do that, but something is preventing it.
Thanks for trying though. It never hurts to have a second pair of eyes.