That is not that easy as it seems. Posts content (i.e. text) goes into the steem blockchain and we host it ourselves (we, as steem community, not steemit). All media; files, photos, videos, are stored in the steem blockchain as links. Their content need to be stored somewhere, outside of the steem blockchain, whether it is YouTube or other providers. It is crucial that Steemit, as currently main gateway to Steem, would provide easy way to add multimedia content, especially pictures, without need to use third-party services. That however would solve that issue only partially (lack of decentralization feature). Maybe IPFS or Storj is a way to go.