If you actually want a preview of the live page, you'll have to use an iframe. If you just want an image preview, Fulvio's suggestion will work, but won't show a "live" preview (i.e., no animations that you would normally see, if the user is logged in to a page, you will only see the front page, etc). It is possible to actually scale the contents of an iframe so that it's a thumbnail, thus achieving the effect you want. For example:
Have fun :)
Copy and paste into your browser's URL bar to preview:
data:text/html,
