Actually, by Creating a "Mask" image. Then using "SmashingPumpkinsAddict" idea, you can actually get perfect Transparent colours, yes colours plural.

Eg. Create a Bitmap half white and half black the same size and shape as your actual bitmap, then XOr this to the background, then XOr your actual bitmap.

This will make parts of your Bitmap "Tansparent"

Hope this helps,
Steve.