'Makes a sample image with two dark and two bright squares.
For example:
e0 e0 1f 1f
e0 e0 1f 1f
1f 1f e0 e0
1f 1f e0 e0'

def make_case(self, mode, size, color):

    case = Image.new('L', size, (255 - color))
    rectangle = ImageDraw.Draw(case).rectangle
    rectangle((0, 0, ((size[0] // 2) - 1), ((size[1] // 2) - 1)), color)
    rectangle(((size[0] // 2), (size[1] // 2), size[0], size[1]), color)
    return Image.merge(mode, ([case] * len(mode)))
