I thought about this too, but one of the features of imageboards is that it doesn't splinter into subthreads, there's a big list of posts, unlike Reddit. And because a post can reply to multiple posts at once, you can't separate them in blockchain forks. If the blockchain forks, it becomes hard to reference posts in the other forks from any particular fork.

On the other hand, there has to be a way to avoid downloading (and sharing) certain parts of the chain, for example if someone uploads illegal content, they should have the option to never download that data, so I like the idea of keeping images separate.

For posting to be feasible, the time to mine has to be low, though of course it'll increase over time, meaning that either shorter blockchains are favoured for ease of use (nobody wants to wait 5 minutes and waste a lot of power just to make a post) but long enough to make them hard to forge.

There's also the issue of segmentation; there's an interest in certain users wanting not to share certain posts, for example people against political issue X may not want to share posts about X. With a small number of peers, this could mean that only one or two peers keeps track of the posts talking about issue X. And then you'd have to trust that you're not downloading illegal content from those people, so if you are committed to anti-censorship but also don't want to download illegal content, you have to trust those peers to only remove illegal content.

In the end, I'm not sure if it comes out better than NNTP, or even centralised discussion boards with multiple independent archive sites available (which can archive posts before they are deleted by moderators).