transparent background in lilypond generated png's

[Stephen Corey]

Hello,

I've been using Lilypond regularly for several years now with great 
satisfaction.  Recently, I've begun using lilypond generated png files in 
powerpoint presentations and I'd like to know if there is a way to 
generate these files with a transparent (rather than white) background so 
I can add something like a textured background image.  Any help is 
greatly appreciated.