[RM2K3] Trouble making a PNG image have transparency

I'm trying to make a Limit Break system for my RPG game, so naturally, I must make custom facesets for this. I'm having a bit of trouble with the transparency though, I'm using Paint.Net and saving as .png 8-bit, but I can't seem to get the transparency to work.

This is the Faceset I'm trying to achieve transparency with. The right side just appears as black.
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