Slice an image into parts slice_size tall.

def _long_image_slice(in_filepath, out_filepath, slice_size):
    """ Slice an image into parts slice_size tall. """
    print 'slicing image: {0}'.format(in_filepath)
    img = Image.open(in_filepath)
    width, height = img.size
    upper = 0
    left = 0
    slices = int(math.ceil(height / slice_size))

    count = 1
    for slice in range(slices):
        # if we are at the end, set the lower bound to be the bottom of the image
        if count == slices:
            lower = height
        else:
            lower = int(count * slice_size)
        # set the bounding box! The important bit
        bbox = (left, upper, width, lower)
        working_slice = img.crop(bbox)
        upper += slice_size
        # save the slice
        new_filepath = _build_sliced_filepath(out_filepath, count)
        working_slice.save(new_filepath)
        count += 1