If it is a pic that is only on your computers hard drive .... you will have to upload it to a web based host such as photobucket.com. Once you have the pic on the web site you copy the site address of the hosted pic in to the "insert image" icon (5th icon from the rt on the bottom row ... kinda looks like an envelope) ... enter it and the pic will post in your reply.
The key is that you can not go directly from your computer to the msg brd ... it must be on a web based host. You can grab a site address of a pic you find on the web already and just paste that address in the "insert image" button
Is that clear as mud?