You cant upload to their server, you need to upload on your side. I use Google + to upload it and I just the link I posted here.
You do the following to add a image to your post.
[img]http://www.google.com/usersxx/image.png[/img]
Also when you a posting something on the top left area of the editor there is an option to add the command for a picture.
change the url to your picture location.