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I recently [webmasterworld.com] had to do this and the answer to your question is yes! The following works in DW4, but I believe that it is no different in MX.
When you go to link to an image/SSI/flash etc. there is an option on the window called Relative to: with two choices, Document and Site Root. Select Site Root.
Using site root will make all your links (once you change this once, it becomes the default) relative to the site that you have defined in your site window.
In Preferences > Preview in Browser, make sure the Preview Using Local Server is checked. This means that when you hit F12 (to preview your work in a browser), the links will render correctly.
HTH