you need to set your avatar separately from your profile picture. The avatar is the one that appears with your username whenever you post.
the avatar can be configured under "settings"
To continue what Tartan explained ... a thread is a discussion that everyone can contribute to. Each contribution is a post to that thread. A blog is more like your own personal area, where you can put anything you want. It can be used like a journal or diary if you wish. No on else can add to your blog directly, although they can make comments on what you've put there.