Also, though it isn't exactly spanking related, there was that Coppertone Girl ad in which a dog is tugging at the bottom half of the bathing suit of the young girl, partially exposing her white cheeks, ostensibly showing the difference between her Coppertone-assisted tanned (not burned!) legs/torso, and her white cheeks. It probably has no effect on people who aren't into spanking, but I always thought of spanking when I saw it (and I was probably just a bit older than the girl in the drawing when I first came across it). I feel a little guilty about this, since the girl in the cartoon was probably about 5.
I once (1970s) bought a souvenir wooden paddle that read "HEAT FOR THE SEAT" and featured a drawing that was clearly based on that Coppertone image; it was obviously supposed to be the same little girl, without the puppy but with her bathing suit pulled down to upper thigh level in the rear, showing off reddened bumcheeks--she was frowning unhappily and had a teardrop sliding down her face, obviously having just been spanked.
The last time I looked at a Coppertone 'water babies' product (sunscreen for children), the girl's suit was still being pulled on by the puppy but it was staying up--I figured that perhaps the change was an overreaction to the possibility of being accused of "kiddie porn" on the company's part...