just load the picture on a hosting website,
http://imageshack.us/ is a good one. They will provide you with a "direct link" copy that link and click "Image" at the bottom of the message box when posting and it will prompt you for the link. Paste the link and you will see it show up as I have it below (it will be lower cased lettering of course so it will work) and the image will show up.
[IMAGE]"web link here"[/IMAGE]
