With regards to links Steemit Inc already remove copyrighted material from the front-end website in certain instances so it's feasible that any app developers may have to comply with similar requests relating to child pornography.
Because pictures are not actually stored on the blockchain (just links) I think the best course of action for governments and authorities would be go after where the images are stored, i.e. the image servers, and require the content to be taken down or isps to block access to them. You would then just be left with broken links.
If it got to the stage where witnesses were required to obscure blocks that contained illicit links I think it would be game over.
The point here is really that images can be stored as text in the blockchain already - you can see this demonstrated in sempervideo's comment at the top of this list of comments (currently). So basically, as it stands, there is no way to prevent illicit material being included into the blockchain.
There isn't and i doubt there will ever be wrt decentralised blockchains.