First, you have to open an account with photobucket.com or some other file sharing site and upload the picture to that site - if the picture you want to post is not already on the internet.
The file sharing site will give you a link to your picture.
Copy that link.
To actually post the picture, click on the box that says URL in it and you will get a little pop up window - paste the link in the pop up window.
That's about it.