Here's another rule.

When you consider displaying a photo, consider whether it's worth risking $25,000 in legal fees and five years in prison to make a few extra bucks.

You'll -probably- be fine, but do you really want to bet your entire business and your freedom that a bored prosecutor won't gleefully cackle, "Oooh, I'll bet we can convince a jury that picture is pornographic!"

--Aaron