I got a square logo and I need to round_corner it, searched for a while and got the follow code "working":

def round_corner_jpg(image, radius):
    """generate round corner for image"""
    mask = Image.new('RGB', image.size)
    #mask = Image.new('RGB', (image.size[0] - radius, image.size[1] - radius))
    #mask = Image.new('L', image.size, 255)
    draw = aggdraw.Draw(mask)
    brush = aggdraw.Brush('black')
    width, height = mask.size
    draw.rectangle((0,0,width,height), aggdraw.Brush(''))
    #upper-left corner
    draw.pieslice((0,0,radius*2, radius*2), 90, 180, None, brush)
    #upper-right corner
    draw.pieslice((width - radius*2, 0, width, radius*2), 0, 90, None, brush)
    #bottom-left corner
    draw.pieslice((0, height - radius * 2, radius*2, height),180, 270, None, brush)
    #bottom-right corner
    draw.pieslice((width - radius * 2, height - radius * 2, width, height), 270, 360, None, brush)
    #center rectangle
    draw.rectangle((radius, radius, width - radius, height - radius), brush)
    #four edge rectangle
    draw.rectangle((radius, 0, width - radius, radius), brush)
    draw.rectangle((0, radius, radius, height-radius), brush)
    draw.rectangle((radius, height-radius, width-radius, height), brush)
    draw.rectangle((width-radius, radius, width, height-radius), brush)
    draw.flush()
    del draw
    return ImageChops.add(mask, image)

then I saved the returned image object,however it has white background in it?how can i get rid of the white background? Thanks in advance~