You just do this :)
[img]IMAGE URL GOES HERE[/img]
For example, if I click on an image of a cat, then all I need to do is copy and paste the URL in between the image code :) Like this
[img]http://cdn.dashburst.com/wp-content/uploads/2013/01/Grumpy-Cat.jpg[/img]
Turns into
(http://cdn.dashburst.com/wp-content/uploads/2013/01/Grumpy-Cat.jpg)
Hope this helped you :)