This should work:
from PIL import Image, ImageDraw
im = Image.new('RGBA', (400, 400), (0, 255, 0, 0))
draw = ImageDraw.Draw(im)
draw.line((100,200, 150,300), fill=128)
im.show()
Basically using ImageDraw
draw over the image, then display that image after changes, to draw a thick line pass width
draw.line((100,200, 150, 300), fill=128, width=3)
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