You need to have it hosted on the Internet somewhere - some people use imagestation.com, but any photo hosting site, or even your own website is fine.

Then, once it is on the Interenet somewhere, get its URL - it'd look something like http://petoftheday.com/archive/2004/October/21.jpg only without the spaces in - so it has to start with http:// and end with .jpg .

Then in your image area, type [img ] and paste the URL then [/i mg] only without the spaces. That's it!