What what you must have done was click on the browse button near the bottom of the reply window. Then you went to a picture that you had filed on your computer and you opened the picture file. When you open the picture file it sends it to the box beside the browse button. After doing that, when you click on reply, as long as your picture does not go over the maximum allowed file size, the picture will be included with your reply. If the file is to large then you will be notified of this and your reply will not be added to the thread. At that point, if you have Photo Shop or Corel Photo, you can modify the size of the file and then re-post.
Hopefully you understood what I was trying to say and this was of some help to you.