masks in ming

Hi,
I'm using MING to make a tool where you can cut your face out of a photo for
use in an animation and it all seemed to be going fine.
Basically I am going uploaded JPEG > GD Image Object.
Then use the GD drawing functions to draw some transparent areas into this
image (based on data gathered from a little flash app).
Then output this image to a PNG which is converted into a DBL which is
turned into an SWF using MING.
All works fine. But.... I just looked at the filesizes and realised that the
final SWF is loads (like 8x) bigger than the original JPEG. I guess this is
because PNG files are less good at compressing photographic type image
data...
So I was thinking maybe I would be better making the PNG without the
photographic data and using it to mask the loaded JPEG. Is this possible
using MING? Maybe it is best to do the masking at runtime but I think that
limits me to the flash 6 player (since I would be loading the image to be
masked and the mask in from actionscript).
Any suggestions,
Thanks,
Kelvin.