How could a filter determine the binary content of an image file? IMO, that an unreasonable request. The only alternative would be to completely eliminate any links or thumbnails to images with nudity tags.
As it is, the moderators will have to view each and every thumbnail, which is a colossal waste of their time (and I feel sorry for them), and only because some of the members aren't responsible enough to abide by the terms of a site providing a free service (for most).
As the owner and site admin for a fairly large racing community for over ten years now, I tend to view most of these squabbles in a different light, and my skin is a good bit thicker than most.
I'm sure that everyone had to agree to abide by the TOS in order for their registration to be confirmed, and remember that free speech only applies to your own web site.

FWIW, JMO, and blast away, I'm used to it.