desired: dynamic syntax for 32 bit with alpha png

Thanks to consistent help,
I am inching closer to the goal.

Attached patch is crude background subtract
that saves sequence of Pngs
using count to increment filenames.

Result:PNG has a black background.

Desired Result: transparent background.

OS : Windows XP, Jitter 1.2.2

I think it is simply a problem with syntax I am sending to exportimage.I am concatinating strings using sprintf and sending "exportimage filename.png png-24 lossless"
How do i specify preserve alpha?

Thanks in advance as always,
Jhave
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