Decentralized forum software has existed since before Bitcoin. The most successful example that comes to my mind is Freenet's FMS. However, since decentralized forums can't have normal moderation, using them generally requires more responsibility and work from readers. The vast majority of people don't want to do this, which is one big reason why decentralized forums are very obscure. (Another reason is that there are very very few developers working in this space, so the tools are often not so great, both from a usability standpoint and in general. IMO Freenet is very unlikely to actually be secure in the face of serious attack, for example.)

Increasing fault-tolerance is a long-term goal, but (re)creating a truly decentralized and uncensorable forum is outside of bitcointalk.org's scope.

The image proxy isn't a caching proxy. Proxied images are never saved to disk, and only small chunks of images are stored in memory at any one time. The proxy does work with client-side caching (it passes on appropriate cache-related headers, etc.), so you may cache images.