for centering, that is the html for it, which also works in markdown mode... <center></center>

For side-by-side: https://steemit.com/photos/@rockymtnbarkeep/how-to-post-pictures-side-by-side

For left or right alignment: https://steemit.com/steemit/@ervin-lemark/how-can-you-left-or-right-align-parts-of-your-steemit-article-images-included

Hope that helps! :)