If you want to upload a picture from your computer: you have to put the adress in the filename box, at the bottom of the "Post a reply" window.
If you want to take it from a website: Ctrl+C (copy) in the adress of the picture [If the pic is not alone in the page, right click in the photo, and click in "Properties", for see the adress of it], Ctrl+V (paste) in the message, select all the adress, and click on the "Img" buton, at the top of the window.
That's all