If its already hosted elsewhere, if you copy the url to the image, will looks something like
http://www.Location-Of-Picture/photo.jpg
Then select the Img button just below the subject line when you post, then insert the copied URL between the Img tags like so
[img]http://www.Location-Of-Picture/photo.jpg[/img]
Will post the pic directly for viewing but is hosted on another site like photobucket.
Regards
Bill