Add transparency on PNGs that have transparency

[MWI]

Hi guys
Back again with a strange question.
Is there a way way to ad transparency areas on a PNG that already have transparancy?

I will try to explain my question.
I have PNGs with a product on a transparent background and want to replace part of the product with the water stamp command.
The new part has another shape than the original one.
Is it possible to first cut out the original part and replace it with transparency and then water stamp in the new part?

I have tried for hours but always get problem with the transparency. Have tried to convert the PNGs to 32 bit with no change in result.
As there is thousand of pictures I want to modify I need to to it in a batch process:(


M

[xnview]

rectangle cut?

[MWI]

Hmm, can't figure it out.
Maybe this pic explains what I'm looking for.
Is there a solution to cut out a piece from a PNG and replace it with transparency background?

BR
M

[xnview]

like that, no sorry