from wand.image import Image, CHANNELS
from wand.api import library

imageOverlay = Image(filename='mona-lisa.png')
imageOverlay.alpha_channel = 'opaque'
library.MagickSetImageOpacity(imageOverlay.wand, 0.2)
imageOverlay.save(filename='test_transparency.png')

from wand.image import Image, CHANNELS
from wand.api import library

imageOverlay = Image(filename='mona-lisa.png')
imageOverlay.alpha_channel = 'opaque'
library.MagickEvaluateImage(imageOverlay.wand, 'multiply', 0.2, CHANNELS['alpha'])
imageOverlay.save(filename='test_transparency.png')