What actually is happening here, is that miners have the opportunity to put a small amount of arbitrary data in a block when they mine. Ideally this is should be random data, its purpose is to change the data being hashed so you get a different hash each time you try.
In my view the author of bitcoin set a bad precedent by including a newspaper headline referring to bank bailouts in the very first block. However I can't begrudge him for that too much. Since 2009, people have included prayers and accusations of illegal activity (trolling) in blocks.
Assuming increased bitcoin adoption, I see mining getting more and more centralized, already I don't think any of the major mining pools would be silly enough to include CP or other illegal content. It would take many blocks to store a non trivial sized image in the blockchain, and if they put a link in instead then of course the host could be taken down.