There's a button in the Reply text box that looks like a rectangle with an arrow coming up out of it. That's the "upload image" button. It gives you two options: one to upload from your computer and one to upload using a direct link from the web.
The first option has been buggy for me, and others it seems, so I just upload the images to postimg.org and then paste the direct link using the second option.
Hope this helps!
