A purely peer-to-peer social network or even something like a photo-sharing website sounds quite tricky. How do I ensure that the node on which the picture I am

A purely peer-to-peer social network or even something like a photo-sharing website sounds quite tricky. How do I ensure that the node on which the picture I am looking for is available? Traditional peer-to-peer networks solved this problem my making redundant copies of files and hoping atleast one will be available. However that may not be a viable solution for private data like pictures etc.