To add an image to the post you need to click on the image icon on the top-right side of you input box.
You will be requested to add the image url between the image tags. To do so, upload the image to http://tinypic.com/ , http://imgur.com/ , http://www.freeimagehosting.net/ , etc.. free image hosting sites. Copy the url of the image along with the extension, '.jpg, '.png', etc and paste it.
At the end of it, it should look something like this