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I should have a look at the masking. Do you know if the png format has an alpha layer for the corners? The drawing function looks as if it should use one if it is present. But I could be wrong - it would help to know if there was alpha there for sure.
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Yes, the png has transparent edge corners (see screenshot below). These show up as white with your current implementation. It looks to me as if the drawing function either doesn't preserve transparency or it might start with a white, non-transparent background and draw on top of that.
In the former case there might be an additional parameter to pass to the function.
In the latter you probably need to make it start with a transparent area before drawing the cards on top of it. It might even be necessary to grab a screenshot of the underlying area and draw that first, then the card image on top of that for fake transparency.
Screenshot of a part of cards0.png in a graphics application at 3200% zoom:
