The reason you see the middle when hovered, and bottom of the image when clicked has nothing to do with the size of the Image, but rather how classic shell uses an image as a start button.
As you can see from this example:
http://www.classicshell.net/forum/viewtopic.php?f=18&t=174the top 1/3 of the image is rendered as the first button state, the 2/3 is the hover state, and the 3/3 is the clicked state.
So In order to make a button you need all 3 images, and the attached only has 1 (I even checked to see if maybe it had layers for the other 2 states, but alas it didn't.)
which means either you could find a matching image for the other 2 states or they would have to be created. (which I may be inclined to do, however would first like to know what you want done for the 3 states)
I would suggest
door closed for default state
door open for hovered state
windows orb glowing for clicked state
OR
door closed for default state
door glowing but closed for hovered state
door open for clicked state
Either one of those I think I could do relatively easily/nicely
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