You can't directly. Yoiu have to upload your picture to an imahe-hosting site and then post it here.
This is the most popular service for doing so:
http://tinypic.com/
(If you need detailed instructions please refer to my post here:
http://www.theanswerbank.co.uk/Animals-and-Nature/Question1426455.html)
However I find this site faster:
http://www.hostingpics.net/
Click the first 'Browse' button.
Navigate to the location of your file and double-click on it.
Click 'Envoyer'
When you see a small version of your picture, click on it.
Then click again on the resulting image.
Copy the URL of that page and paste it into an AB post.
Easy peasy!