im = Image.open('mouse.png')
im = im.convert('RGB').convert('P', palette=Image.ADAPTIVE)
# The white index in the palette of this image is 255
im.save('mouse.gif', transparency=255)

import Image

old=Image.open('test.jpg')
new=Image.new('RGBA',old.size)
mask=Image.new('1',old.size)

for x in xrange(old.size[0]):
	for y in xrange(old.size[1]):
		rgb=old.getpixel((x,y))
		alfa=1
		if rgb==(255,255,255):alfa=0
		mask.putpixel((x,y),alfa)
				
new.paste(old,mask=mask)
new.save('new.png')

            xsize, ysize = im.size
            back = Image.new('RGBA', (xsize,ysize))

            source = im.split()
            #R, G, B, A = 0, 1, 2, 3
            mask = im.point(lambda i: i == 255 and 255)
            source[3].paste(mask)
            mask = Image.merge(im.mode, source)
            im.paste(back, (0,0), mask=mask)

            xsize, ysize = im.size
            back = Image.new('RGBA', (xsize,ysize), (255,255,255,0))
            for r in range(5):
                mask = im.point(lambda i: i == 255-r and 255)
                source[3].paste(mask)
                im.paste(back, (0,0), mask=source[3])

source[3].paste(mask)

im.paste(back, (0,0), mask=mask)