To center it you will have to go to the markdown editor and use the center command. Markdown is a bit trickier but if you learn how to use it your posts will go to a whole new level. Here is the link for howcto use it =) hope it helps. https://steemit.com/steemit/@scrooger/pimp-your-post-2-or-formatting-and-editing-your-blog-made-easy-or-cheat-sheet-included-or-just-copy-and-paste
Thanks for the vote and follow =)