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You can try GIMP's Color > Color to alpha. You must combine with layer masks
one version which is otherwise well separated from the background but the wings are still green
another version where the wings are transparent as wanted but something else green has also vanished
Hopefully you have a high resolution photo. The shown version in the question is ...
1
...As for your second question, when you remove pixels in Photoshop using a mask, it still keeps pixel data. For example if I remove a part of an image
and then run Layer > Layer Mask > From Transparency and hide the mask you'll notice that all the pixels were restored even though they aren't presented in the original layer:
And when you save PNG24 ...
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