And why? If your site definition is properly set up, all links will be relative and there is no need to add anything. Everything else is out of your control - if you link external images from another server, DW cannot know about them and only manualyl inserting the full URL will make it aware of the file.
It's for a project I'm working on. In normal circumstances I understand this is not needed but what I am doing is creating an HTML template page with images/flash then coping the HTML code to another site so all the images need the full URL.
Just looking for a quicker way then manually adding the HTTP:// to each graphic.