Tell me the image url you want.
There are many possible errors such as :-
You give a non-direct link with redirects
You don't give a image link, but a site link
You upload to an unsupported image server.
I suggest imgur, and the URL should always be
i.imgur.com/(code).jpg (jpg may be something like png instead)
Then you can use
[img]i.imgur.com/(code).jpg[/img]