First of all, you can't post directly from your hardrive like you would send an e-mail attachment. You must first upload the picture to the Web somewhere using any one of the number of free image hosting sites (such as Photopoint, Angelfire, Yahoo, etc.)
Once you have done that, then link the picture by posting the URL (to get the URL, go to the picture on the Web, right click on it, go to "Properties", then copy the location)
Then come here, and paste the URL. But be sure to put a "<" and "IMG SRC=" BEFORE the URL, then put a ">" at the end of it. Then post the message.
The picture should then appear.