FFMPEG How do I render a FLV from multiple PNG while saving the alpha channel?

FFMPEG How do I render a FLV from multiple PNG while saving the alpha
channel?

I'm using FFMPEG, and when I'm rendering a FLV movie from multiple PNG
files. I would like to preserve the png's alpha channel (transparency) in
the output FLV file.
If you could please help me out on this one, I would appreciate it.
Thanks, Dan.