Hey nmai - this has probably been answered before, but what you need to do is to upload your picture to an accessible website, then use the IMG button to post a link to it.
The images are not hosted on PPRuNe itself - you have to provide the host site.
So if your picture is called "picture.jpg" and you've uploaded it to a website called "www.nmai.com" the link would look like:
[_IMG]http://www.nmai.com/picture.jpg[_/IMG]
but without the underscore ("_") characters.