Why doesn't PIL free memory after opening some png files?

Question

Why doesn't PIL free memory after opening some png files?

Let's say I have a service that gets images from the internet and stores them in a directory. I receive the input as .png files and save it locally. I am using PIL to open images and add a background to them as needed. I recently noticed high memory that occurs when processing some .png files. It seems that when opening some .png, the PIL does not free the memory properly.

The following code will demonstrate what is happening:

from PIL import Image
import resource

def check_mem(filename):
    mem = lambda:resource.getrusage(resource.RUSAGE_SELF).ru_maxrss
    print mem()
    img = Image.open(filename)
    print mem()
    img = img.convert('RGBA')
    print mem()
    background = Image.new("RGBA", img.size, (255, 255, 255))
    print mem()
    image = background.paste(img, img)
    print mem()
    del background
    del img
    del image
    print mem()

if __name__ == "__main__":
    import sys
    check_mem(sys.argv[1])

For some images (for example, images like this one ) this generates the following output:

You can see that the memory is almost doubled! And even at the end, when I deleted all objects, the memory is still twice as large as it was at the beginning.

For other images for an example of this , the memory usage does not change: