To attach a pic to a reply, instead of using the quick reply box at the end of the page, just use the post reply button http://www.ssnakess.com/forums/images/reply.gif and you can upload your pic just like when creating a new thread.
Also be sure to check out the <a href="http://www.ssnakess.com/forums/forumdisplay.php?s=&forumid=63">How to use the forums</a> forum.
Hope this helped!