First, you have to upload your image somewhere...somewhere that gives you webspace, such as Imagestation.
Then, you hit the "IMG" button and type in the URL (link) for that picture. Then "close" the link by hitting the "IMG" button again.
For example, I want to post a pic of my car, here is what I would do:
[ IMG]http://www.amirsaleh.com/G35/Zaino01.jpg[ /IMG].
But you would type that line without the spaces in the "[ IMG]" and "[ /IMG]" brackets. Here is what that line of code does: