Get the URL for the picture you want to post in the thread, or wherever - it should start with "http: " and end with ".jpg" - then type [img ] (without the space, paste the URL in, and type [/img ] without the space.

So for example, without the spaces, this:
[i mg]http://Catoft heDay.com/arc hive/2012/August/09b.jpg[/i mg]


Would look like this: