Center an Image

1. In HTML5, put img tag like this:
<IMG class="displayed" src="..." alt="...">

2. Then CSS3 should be:
img.displayed {
    display: block;
    margin-left: auto;
    margin-right: auto;
}

3. If you want to dynamic it in php then use image like this [MIND THE CLASS OF displayed!]:
<?php the_post_thumbnail('blog', array('class' => 'img-responsive displayed')); ?>