I'm not sure, Tropical. I think
it might be that the URL has to end in jpg, jpeg, or gif, in order for it to post as an image. I think png works, too. There may be other formats that work, as well, but those are the ones that come readily to mind right now. (I'm still working on my morning coffee.
When I have a specific
pic that I want to use, and I can't get it to display on the forum, I download it to my computer, then upload it to an image host, like Photobucket. Then use the IMG code they give for it. I only do that when I'm having a hard time finding a pic that I feel clearly illustrates what I'm trying to describe. If there are a lot of images available that would serve my purpose, I just keep looking until I find one that's in the right format.
I'm not the most computer savvy person around. Maybe someone else will pop in with a good answer for you.