I think that I might have been wrong about part of my suggestion. Rather than dragging it to the Quick Launch, try right-clicking it and select "Pin To Taskbar."
I just deleted my own 'Show Desktop' shortcut from my Taskbar to test things, and recreated one using
Option One, where I download the actual shortcut to my Desktop from
HERE.
I don't know if the proper icon was suppose to be there when I downloaded it, but it wasn't, so I went ahead and created a new one using Google Images. I just searched for the 'Show Desktop' icon. It will need to be in an .ico format. If you don't have any photo editors that will change it, then simply google .ico conversion websites and there will be a bunch of sites to change it for you.
From there, just place it where you want, then when you want to change the icon for the shortcut, right-click it and select "Change Icon." Now just navigate to where to put it. (I just pop a copy into System32 myself) Now right-click and select "Pin To Taskbar" and that
should fix things.
With that said, the correct icon might actually be in the icons folder, (which I would suspect that it is) but I got tired of looking. Plus, I like the older style anyhow.
b1rd