Your code should be something like this:
<layout id="normal" background="player.background.noalpha" alphabackground="player.background.alpha" desktopalpha="1">
player.background.alpha - this is the image that has the dropshadow on it.
player.background.noalpha - this has no dropshadow on it, and no partially transparent areas on it.
Winamap switches between these when deskopalpha is enabled or disabled in the preferences window.