there's 2 ways to do this:
1st way.
(from the gallery)
1: click on gallery in the menubar and click "upload picture" on the dropdown menu.
2: click browse, choose a picture from your computer, fill out the rest of the info, choose a category and hit upload.
3: When the picture is uploaded copy the BB image code:
4: add the BB image code to your reply.
2nd way.
(from another site)
1: click on the picture with your right mousebutton and select properties.
2: copy the URL adress of the picture.
3: In your reply click the "insert image button" (the yellow square with the mountain in it).
3: paste the URL adress into the pop-up that appears.