You have to upload it onto a free site in the internet first, and then you can add a link to the picture with the help of the image button.
I never did it myself, because I have my own homepage.
Search via Google for "free uploading for picture" or something like that!

BTW: Take a look at the tutorial forum!
There I found for instance this tutorial for Med:

http://exile123456.ex.funpic.de/

And there may be others, too.