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A possible parallel solution using `imageio`:
```
from imageio import imwrite
from multiprocessing import Pool
from numpy import load
from pathlib import Path


def npy2png(npyFile):
    imwrite(npyFile.stem + '.png', load(npyFile))


rootPath = Path('/path/to/Main_dir')
if __name__ == '__main__':
    with Pool(processes=8) as pool:
        pool.map(npy2png, rootPath.glob('*npy'))
```

Additional arguments can be passed to `imwrite`; in the case of the PNG format, execute
```
import imageio
imageio.help(name='png')
```
to know which ones are available.