Loup Garou,
I have no idea what you attempted to do but I have a test Board on my home machine where I have created a user who is a novice (pilot fish) and 'he' can do just as I stated.
Now, I am sure that if you can see the "Img" button at the top of your editorial window then you have the permission to use it successfully.
This is the text of your link:
"http://www.famouspictures.org/images/a/aa/BettyGrable.jpg"It merely needed you to highlight that link and press the "Img" button and then you should have seen:
[img]**http://www.famouspictures.org/images/a/aa/BettyGrable.jpg**[/img],except that I put in "**" to corrupt the link so that it displayed as I 'required'.
Here it is without corruption:

Try it using the "Preview" button first.