There are instructions up top. Basically you need to click on your picture until you get to a page where it's just the picture only. The URL will end with .jpg, copy that URL.
Here, when answering the post, you'll see a little icon of a sun and mountains in the editing box. Click on that, paste the URL there. Then I always set the width to 600. (There are other ways to do this, but this works for me). Put in some text, and you're done.