A background image may be possible, and would be very nice. I agree with that much. Transparency, however, is not an option unfortunately. There's some issues with the windows OS and such that limit, or prevent it entirely if I recall correctly.

Even when it does work, it makes it so I think BYOND only works in hardware mode and it's normally in software mode? I forget. There's two modes, and one becomes unavailable with transparency.

Things like this are why it's only possible to do transparent outputs, and even those can be somewhat problematic with straining the game.