Flickr and Tumblr have more in common than just the inability to buy a vowel. If the kind of things I've seen on Flickr have been allowed to exist for years under the Yahoo umbrella, I can't imagine that Tumblr's risque content is in any danger.
Here is a good (and mostly SFW as far as images) Flickr group that advises on how to work within Flickr's terms of service: http://www.flickr.com/groups/unsafe/