Guys, in decentralization if you mean that no post will get removed and no topic or subject will get banned, then keep in mind that many people might also post some illegal and unwanted things. That will bring more problems.
Of course, for that reason peer-level-moderation is needed. Which means that yes, some content can be removed, and even should be removed. And who should do that?
Theymos wrote about this a while ago: something like the DefaultTrust system voting could work. But without some moderation and Merit requirement, DefaultTrust could be overrun by a 51% attack. So when it's fully decentralized, that may happen.
Is there any visible (not encrypted or external link) pedo pr0n jpeg in the BTC blockchain?
If so, all BTC nodes are illegal by definition.
Bitcoin nodes don't show images, and the Bitcoin blockchain doesn't support images. Anyone can give an external meaning to data, but that meaning is irrelevant without using external software. And if you want to search for illegal images, there's probably easier ways than searching the Bitcoin blockchain.