def pick_orientation(img1, img2, spacing, desired_aspect=1.618):
    """Pick a tiling orientation for two images.

    Returns either 'lr' for left-and-right, or 'tb' for top-and-bottom.

    Picks the one that makes the combined image have a better aspect
    ratio, where 'better' is defined as 'closer to 1:1.618'.
    """
    w1, h1 = img1.size
    w2, h2 = img2.size
    size_a = w1 + spacing + w2, max(h1, h2, 1)
    size_b = max(w1, w2, 1), h1 + spacing + h2
    aspect_a = size_a[0] / size_a[1]
    aspect_b = size_b[0] / size_b[1]
    goodness_a = min(desired_aspect, aspect_a) / max(desired_aspect, aspect_a)
    goodness_b = min(desired_aspect, aspect_b) / max(desired_aspect, aspect_b)
    return 'lr' if goodness_a >= goodness_b else 'tb'
