the problem is that when I press the show desktop button it show a different image than what it should
The show desktop section is build up with "three images" if you use one and dont change the value it will take parts of the skin you are building your skin on.
there should be also a box that says "image count 3" ( look at the screenshot )
ALso make sure that the alpha channel is premultiplied is - not checked
Then go under extra tab and set " blend against taskbar " to enabled ( if the image has transparency )
Edit: in Windows8 you will only see it if you click it ( normal and mouseover do nothing )