To fist post a picture it first needs to be on the web somewhere. If the picture was on your website for example you would copy that link location and put it in-between a
Like this:
If your picture isn't on the web then click on the button. This will bring up a menu, click browse... and chose a file from your computer. Then click the host it button. It will then bring up another menu with three different boxes. It's best to choose the middle one, so click copy next to that box and it will ONLY highlight all the text in the box. So you need to right-click on the highlighted text and copy it. Then click the button next to hosting button: and paste the copied text in to text and press OK.