抠 literally means 'to dig/pick out with a finger', so 抠图 is 'digging the subject out of the image'. The mental picture of plucking someone off a photo maps nicely to the editing gesture.
culture
抠图 is also a pejorative in Chinese entertainment news - 'green-screen acting' / 'pasted-in performance', used to mock dramas where lead actors film their scenes separately and are composited in.
