You can't do that. The image has to been hosted on a server somewhere. Get an image host account (photobucket, imageshack, flickr, etc... preferred image hosts is another big topic for another thread) then upload the pic to that and swipe the appropriate code to post in forums.
I'm a little surprised with all your IT experience and knowledge of computers (way beyond my own I might add) that you aren't aware of this.
Are you yankin' my chain? If so it's working. ;-)