The other simple alternative would be to just go all Stalin on the membership and declare no nudity in thumbnails, period, and ruthlessly delete them along with the image, and even make the offender take a few days off.
This is what I would do, to avoid this sort of thing entirely.
Sure, they'd whine and cry at first, but they'd get over it.
Besides, all this suggestive content in the thumbnails is only a ploy to get hits on the images, and then sit bake and rake in the gushing praise, which in and of itself is kinda silly too.