Magic!
Or, on a more serious note,
you can either upload the photo to the post (at the bottom of the post, underneath the icons, there's an 'attach files' box. Click on that, upload and you should be sorted)
or you can use a web-based host like
http://imgur.com, and copy the img code (it'll be in bbcode - to use on this website it'll be a link that looks something like [img.]http://imgur.com/xFYB9[/img.]. You then copy and paste that code into the window, and the image should then appear.
