If you scroll down from the message box, you'll see a button labled 'Manage Attachments', click on this and a new box will appear.
In the space provided, type the full path of where your pic is located on your PC, or click on the browse button and find it that way. Once you have done this click the upload button. Bear in mind that there is a limit on the size of pic you can post... it can't be wider than 800 pixels or taller than 600 pixels, and the file can't be bigger than 100K (if need be save it at a lower resolution 72-150dpi on your PC).
Once you've done all this close the 'manage attachments window, and then click 'submit reply... That's it... just sit back and wait for the replies!!!