Of those, how many do you think are able to ensure that none of their followers have been previously identified as highly-connected nodes in the underground networks dedicated to sharing child pornography, and therefore pretty much exclusively post child pornography or follow people who they believe are likely to post child pornography?

Of those, how many do you think are able to ensure that none of their followers ever repost/reblog that photo on any other blog which also happens to contain at least one other photo that's been previously identified as child pornography?

How many people do you think have Tumblr accounts where they post only new, never-before-seen pornography of underage children, which has never been posted on any other blog before, and never once post a single photo that has been previously identified as child pornography? So if Tumblr had a problem with original content (which could include underage users posting explicit pictures of themselves, as well as the worse things we think of when we hear the phrase 'child porn'), I don't think that would help. > Maybe I'm misunderstanding, but PhotoDNA seems to be a tool for identifying reposts and edits of already known illegal images - those that have previously been found and had their hashes entered into the database. They were doing this for years, before they were even acquired by Yahoo. Of course they were working with NCMEC (and therefore using PhotoDNA, which is owned by NCMEC) it would have been legal suicide for them not to.

So it looks like they might have already been using PhotoDNA.

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