I don't think the pictures or videos or whatever are uploaded to the blockchain.
What's uploaded is the body of the message with text and links to those images.
I haven't really checked where the images are going, but if it's something like dtube, it would be stored on IPFS (not blockchain). https://steemit.com/dtube/@viewangle/dtube-where-are-the-videos-stored

Nevertheless, with so many people's privacy groups in the world, it's only a matter of time until some extreme case comes up. Privacy law in many countries gives you the right to delete all your information from a specific online site, so if that is not possible, some court might want to either ban or go after the witness servers to try to shut down the system.

A joint collaboration between US and Europe (think piratebay) would probably be enough to shut down most witness servers... thus exchange would be affected heavily too.