How to extract image with alpha from several screenshots with known background and image with alpha on it (not very precision)

ImgExtract.py

from PIL import Image
im0 = Image.open("IMG_1.PNG")
im1 = Image.open("IMG_2.PNG")
im2 = Image.open("IMG_3.PNG")
im3 = Image.open("IMG_4.PNG")
size = im0.size
pix0 = im0.load()
pix1 = im1.load()
pix2 = im2.load()
pix3 = im3.load()
bkcol0 = pix0[5,50]
bkcol1 = pix1[5,50]
bkcol2 = pix2[5,50]
bkcol3 = pix3[5,50]

print bkcol0, bkcol1, bkcol2, bkcol3

def solvecol(c0i, cb0i, c1i, cb1i):
    c0 = float(c0i)
    cb0 = float(cb0i)
    c1 = float(c1i)
    cb1 = float(cb1i)
    a1 = (c1 - c0) / (cb1 - cb0)
    a = 1 - a1
    cc = 0
    if a > 0:
        cc = (c0 - cb0 * a1) / a
    return ((a*255), cc)

def solvecolor(c0, cb0, c1, cb1):
    rc0, gc0, bc0 = c0
    rc1, gc1, bc1 = c1
    rcb0, gcb0, bcb0 = cb0
    rcb1, gcb1, bcb1 = cb1
    ar, r = solvecol(rc0, rcb0, rc1, rcb1)
    ag, g = solvecol(gc0, gcb0, gc1, gcb1)
    ab, b = solvecol(bc0, bcb0, bc1, bcb1)
    a = (ar + ag + ab) / 3
    return (r, g, b, a)

imr = Image.new('RGBA', size, (0, 0, 0, 0))
px = imr.load()
bkcolr = px[5,50]

for x in range(size[0]):
    for y in range(size[1]):
        k = 0
        if pix0[x,y] == bkcol0:
            k = k + 1
        if pix1[x,y] == bkcol1:
            k = k + 1
        if pix2[x,y] == bkcol2:
            k = k + 1
        if pix3[x,y] == bkcol3:
            k = k + 1
        if k<3:
            r0, g0, b0, a0 = solvecolor(pix0[x,y], bkcol0, pix1[x,y], bkcol1)
            r1, g1, b1, a1 = solvecolor(pix0[x,y], bkcol0, pix2[x,y], bkcol2)
            r2, g2, b2, a2 = solvecolor(pix0[x,y], bkcol0, pix3[x,y], bkcol3)

            r = int((r0+r1+r2)/3)
            g = int((g0+g1+g2)/3)
            b = int((b0+b1+b2)/3)
            a = int((a0+a1+a2)/3)

            px[x,y] = (r, g, b, a)

imr.save("rslt.png", "PNG")