I think one of the beautiful parts about Steemit is that we don't really know what it is yet. I think it's closest to Reddit if anything, but it's still infantile enough to become much, much more as time goes on. I can't say I disagree about people using it like Instagram or Imgur, but that's just its nature for now. Perhaps eventually there will be separate components to it where images and longer-form text posts can be submitted. For now let's just be excited it's being used!