Copy/pasting images is not working properly on Windows

[CelticMinstrel]

The Edit Custom Sheets option in the scenario editor lets you copy graphics from sheets or paste them into a sheet. However, on Windows, the copied images don't work correctly when pasted into other programs - from memory, I think they paste okay into Paint.Net (possibly missing the alpha channel), but GIMP can't even detect that there's an image in the clipboard.