[natrius]

I don't use Tumblr, but my understanding is that a user has to upload an image for Tumblr to host it. The DMCA safe harbor provisions apply in such cases. Pinterest itself downloads images from pinned items, which probably makes them liable for the infringement.

[tomg]

Interesting... I was unaware that a relatively minor technical difference (browser POST vs. server side GET) mattered so much.