You will want to go to a website like Imgur (https://imgur.com/upload). Here you can upload the picture you'd like to post. When the upload is finished, you press RMB on the image and click 'copy image address'. This link you can paste in a reply on these forums. Add [img] before the link. And [/img] behind the link.
Edit: Actually they've altered it a bit. You can press the 'insert image' button you see when posting. Pastle the link into the blank after 'URL'. Then the tool will take care of it.