I believe that the HoverImage and PressedImage properties of ImageButtons inside a BillboardGUI don’t work in any situation. It does work for ScreenGUIs but not for BillBoardGUIs.
When put in a BillboardGUI:
These are the properties of the ImageButton inside the BillboardGUI:
As you can see, the HoverImage and PressedImage both have a proper value.
It doesn’t seem like it’s a major problem. It’s possibly a bug. I’d maybe experiment around with it for a while and see if anything else can fix the problem.